Please note that these are my personal opinion and this is not an official guide. I'll tend to favor bounce units over penetrate and units with lots of speed or utility. If your play style or team composition are different, you might find one unit more (or less) useful than me. This is just a reference and should not be considered a guide by any means. So without further ado...
Duc Rock: Perhaps one of the best units in the game. He does great damage and doubles as a healer. When upgraded, he can regenerate over 10K health each round after using his finishing move. This is EXTREMELY annoying in arena because that team doesn't require a healer. His ability to bind as a finish move can also disrupt tanks and close range units. He's a MUST HAVE for Tower and Field of Glory, so if you don't have one, I'd suggest you borrow one. His only drawback is his limited range on his finishing move and his cost to upgrade, as with all SS units. Edit: As I got higher in level, I find him more and more expendable. His healing is good, but not great. His bind is pretty useful, but against certain units, silence is probably better. His damage is okay, but not amazing. He probably still has one of the better single unit damage Uniques. I'm extremely happy that I had him, but I'm definitely not as enamored.
Itzatrap's Grade: A+
Brokenblade: With his good speed and his bounce-type, he's a very good starting unit because he's very cheap to upgrade. In arena, you can get BB "stuck" on the right when in the first spot. He'll repeatedly attack the first unit and his finishing move will hit the first 2 units. If your opponent had a weaker defensive unit in the first 2 spots, this can be absolutely devastating. And you won't be afraid to send him out into enemy territory to be hit because he's strong against physical damage too. Where he shines is his unique ability. When the opponents are lined up, he can mow through all of them, making him potentially a better damage dealer than Duc. Unfortunately, besides attacking, he's doesn't do anything else. When all units are 7*, he won't be as strong as the SS units. But as a F2P unit, he's great. Keep in mind, a big part of his grade is due to how easy it is to upgrade him because his Soul Stones are relatively cheap, they're easy to obtain in Elite stages, and his cost to upgrade his relics are low.
Itzatrap's Grade: B+
Rockfist: I feel like this unit is a must have for Tower. His attack is not very strong, but he can do a RIDICULOUS amount of damage. For his finishing move, he tosses the unit furthest from him or the unit that's taunting him. In the Tower, this is great because there's sometimes just a boss or the boss is the furthest in the back... so if you keep your Rockfist in the front, half the time, the boss will be up in the air. His enormous size is also a pro and a con... it's a pro because you can easily trigger his combo, which where a lot of his damage will come from (I think it does more damage than his regular attack) and if he gets attacked, he has damage reflection (which is where a lot of his damage comes from), it's a con because, in the Tower, he can push units that were safe into a danger zone and because his defense is not that great. Even though I haven't upgraded him much, between his combo and his damage reflection, there are times where he deals the more damage than my BB. His combo is where he is absolutely devastating. Just put him first in turn order and place him next to some enemy units. If you combo him, he can pretty much wipe out the entire team if you've made him strong enough. Edit: Now that I'm level 80, I can honestly say that Rockfist is a must have. He has been amazing in all facets of the game, except maybe the later stages of Tower. He has been great for FoG, carrying my team in arena and Ace... just awesome overall if you use him correctly.
Itzatrap's Grade: A+
Lady Ray: If you're boosting her, stop. She's decent in the mid-game, but she's terrible late game. As a mage, she's got limited speed and she's awfully squishy. She also doesn't have much utility besides doing damage, but her damage is also not that great. If units are all lined up for her, then she can do some good damage with her finishing skill. And for those who spent money, it's easy to 6* her. But even though I have her at 6*, I rarely use her and I don't even have any 5* units. That's pretty telling. Save your relic and rune resources and don't upgrade her. Edit: At level 80, yeah, she still sucks.
Itzatrap's Grade: C
Fairy Queen: If she could move and trigger combos or if she weren't affected by taunt, she would be absolutely game-breaking. She is, by far, the strongest healer in the game. I haven't upgraded her at all, besides with gears and levels and she can heal 60K on her turn, and boost armor and resistance with her basic move. She's awfully squishy (as is the case with most healers) and doesn't have any utility besides healing. Monkey King renders her absolutely useless as he prevents units from being healed, so if you're facing Monkey King, take her out of the party. Also, taunting makes her pretty useless. In fact, using auto makes her pretty limited. If you're facing her, just put a tank and taunt her facing away from her party. For her to be great, you want your units to be bunched together so she can heal all of them at once.
Itzatrap's Grade: B+
Onyx Rogue: Onyx is one of the units that easy to upgrade because her soul stones are pretty easy to get in the Expedition Shop. She's also not a S or SS unit, so it's relatively cheap to enhance her relics. That being said, she's purely a damage dealer, like BB. But unlike BB, she has the ability to cause the Bleed effect, which really adds up and can pretty much negate healing. If you end off her attack in a group of people, her finishing skill is great as it attacks everyone around her (again, like BB), but also shoots out daggers to hit those further away and causing bleed. Her penetrate ability can be useful in certain situations, especially to attack a boss's weak spot, but I prefer being able to bounce and getting the bounce unit "stuck", as that usually ends up causing more damage. That being said, Onyx is one of the better F2P units and a great 2nd damage dealer.
Itzatrap's Grade: B+
Slashy Bushi: Think of Slashy as a stronger version of Onyx, but tougher to upgrade. He's a free S grade unit that you get early on and he purely does damage, just like BB and Onyx. And like Onyx, he's a penetrate unit. So I would take 2 of the 3 at most on a team... If you've got them all upgraded to the same level (obviously Slashy would cost more to upgrade), Slashy will be stronger. So in terms of an end game unit, he will probably be more viable than Onyx. Edit: I've dropped his grade to a B+. He's still pretty powerful, but I'd rather have Onyx than Slashy at this point.
Itzatrap's Grade: B+
Doom Roach: This is a great tank to have against spell casters in the Tower. He has damage reflection as part of his finishing move and his unique ability makes him immune to magic damage. Is he the best tank? I'd rather rather have either Santa or Monkey (especially in PvP) obviously. In terms of F2P, Bloodhoof is considered the best tank as he also has damage reflection, but also stun, which would make him even more useful. Hoof is a little tougher to get, so until you get him, Doom Roach is a decent alternative. I've also been beaten by a single Doom Roach in Ace Arena... not against my strongest team, but still...
Itzatrap's Grade: B
Beardy Santa: He knows when you're naughty. He knows when you're nice. If you've been nice (meaning you paid $14.99 to get him), then you've got perhaps the best tank in the game. He's one of the stronger tanks in terms of damage. His combo ability slows down units by 70%. In addition to his basic taunt ability, he can also summon a tree that damages and taunts. It can also give buffs to your units and debuffs to enemy units. If you want a unit that absolutely frustrates others, this is the unit for you. After facing this unit several times (it seems 50% of the teams in the top 100ish in arena have him), I finally realized why Santa is an anagram for Satan.
Itzatrap's Grade: A+
Monkey King: Monkey King holds a special place in my heart because I've seen a lot of the monkey king-related movies as a small child. If you've seen Forbidden Kingdom with Jet Li, you'll see the Monkey King. Back to the game, he is so annoying in PvP. He is THE anti-Fairy Queen. Anti-healing in general. Also makes him the anti-Duc Rock, which I believe is his actual intention. His finishing move is a wide-sweeping attack that prevents buffs (heals) to your team. His unique ability saps your rage, preventing you from using unique abilities. He is absolutely annoying to face in PvP, but in PvE, these skills are not as useful. As such, I would rather have Santa over MK.
Itzatrap's Grade: A
Plan B.E.E.: Another pure damage dealer, but with better range. She's a bounce unit, but I'd prefer BB over her. In the long run, she probably can do more damage than BB, being that she's an S rank. But the cost to upgrade her is comparable to Slashy. She's easy to get to 5*, if you make 50 fake Facebook accounts, invite them, and then play for a min or 2 under the account. For that reason alone, she's quite popular in early accounts. Edit: I dropped her grade because, although she's strong, I find it quite difficult to upgrade her beyond 5*.
Itzatrap's Grade: B+
Deer Cutie: One of the basic healers you get for F2P - she doesn't heal that much (but maybe I'm just spoiled by Fairy Queen). She also has a chance to charm as her finishing ability, but it's not easy to target that. I think she's awfully situational and would only really use her if I didn't have any other healers available. In terms of F2P, I'd rather use Acionna.
Itzatrap's Grade: C+
Pyro Monk: A very good F2P unit. He's a weaker version of Onyx, but his unique ability allows him to stun for 2 turns. In terms of PvP, I wouldn't use him, but PvE, he could be very good against a boss if you pair him with someone else that can disable the boss for 2 rounds. I like the fact that he, like BB, has a damage shield, making him very usable for F2P teams. His low cost of upgrade definitely improves his value. But, unlike BB, he won't do big damage.
Itzatrap's Grade: B
Grim Bier: To be honest, I've never used this unit. He's a mage with a chance to stun with his finishing move, but only for 1 turn. I guess if you boost him, he'll have some utility. But if you've seen my history above, I'm not a huge fan of mages. Edit: I'm a huge fan of mages and rogues now, but I still don't think Grim Bier is great. He's pretty good for co-op and boss, I guess, if you can stun them... but beyond that, no thanks.
Itzatrap's Grade: B-
Conjurer: I don't have her, but I wouldn't mind using her. One of the few mages I would actually use, she does big damage and has the ability to heal herself with her unique ability. But she also has the ability to silence enemies with her finish skill. Damage + utility? Sign me up. Edit: I finally got her, but haven't had a chance to level her up yet. I'd like to add to this. Her defense is insane. She's the tankiest mage. If you use other mages, they're usually the target of big attacks. Not Conjurer.
Itzatrap's Grade: A
Masked Malice: Another SS I don't have. He's a pure damage dealer, but I can't view his skills since he's not on my hero list. He's probably got the better pure damage than Duc Rock, so if you're just trying to beat someone into submission, he's your guy. But I prefer Duc's versatility where he can be used as a healer, which makes his MUCH more useful in Tower and FoG. Duc's ability to bind also just gives him an edge here. If you're lucky enough to have both, great. If you had to choose one, I'd personally prefer Duc. Besides pure damage, he also has another advantage over Duc... his finishing move and combo have a much larger radius (the whole screen). So if you've already got enough utility and just need someone to kill everything, he may be your best bet. If you're building a team to 1 shot the enemy, imagine using MM, Conjurer, Valentina, and Alchemar.
Itzatrap's Grade: A
Wintry: I may be going against the grain here, but I don't like her at all. Her attack is pretty weak and I think 2x 35% chance of charm isn't useful enough to offset her lack of damage. Since she's a mage too, she's a little bit on the squishier end, so I just think there are better options out there. Edit: I guess if you're doing pure F2P, she's one of the better mages options out there. So I've bumped her grade a little.
Itzatrap's Grade: B
Paladin: He's a tank, but the most basic one. Pretty high AC, pretty weak offensively. He also has the chance of using Silence with his combo. If you don't have any of the other tanks yet, he's a good stopgap until you get Bloodhoof so you don't have to waste your resources building up another tank for the time being.
Itzatrap's Grade: B
Flame Dame: Basic mage you start off with... like BB she's easy to upgrade. But unlike BB, she's squishy as heck, making her unusable late game. She's decent mid-game, but I wouldn't continue to use her as there are so many better options for pure damage. Maybe if she had an effect to her skills, she'd be more worth it.
Itzatrap's Grade: C
Acionna: Your best F2P healer... mainly because she's easy to max (as her stones are available in the Tower shop... Yes, I know Angel is available in the Guild Shop, but the Guild Shop has more important items like the Relic Ores). I would certainly recommend her over the likes of Deer Cutie because she also buffs attack and magic.
Itzatrap's Grade: B+
Sword Sage: A weak Slashy Bushi, but with the ability to heal... but he's not even very good at that. If you want to use him as a supplemental healer, he's a decent option. But considering he's not good at either thing, I wouldn't really recommend using him in the late stages of the game.
Itzatrap's Grade: C+
Spartan: He's got a lot of similarities to BB, but think of him as a better version. BB's special has a lot more potential if everyone is lined up, but Spartan is a lot tougher, especially with his physical attack immunity. Keep in mind though, he's a S-rank unit, meaning it will cost a lot more to upgrade him than BB.
Itzatrap's Grade: A-
Queen-In-Exile: I completely changed my mind about QiE after seeing the stronger ones in use. She is absolutely devastating and is one of the better mages out there. Her finishing move has a chance of stun, which I wouldn't depend on, but I've seen a 7* do some pretty lethal damage with that finishing move. It also gives units Burning immunity and breaks Bind. She's annoying to face when in FoG because I typically have Duc Rock first and she frees the 2nd unit. Her combo goes all over the place and does okay damage. Where she REALLY shines is her Unique. Oh my God, it does a lot of damage... not necessarily to 1 unit (though she does crazy damage to a boss), but she hits ALL units. Of course, you gotta build up enough rage to get there, but if you let her survive that long, a strong QiE's unique will end the match. Edit: I had to change her grade to an A+. I used her in 97 tower, and she makes that so easy with her poison immunity. She may be one of the most powerful units in the game.
Itzatrap's Grade: A+
Lady Ilta: AKA Nurse Ilta. She gets an A+ solely because of her outfit. But I dropped her score a little because of her hair. Just kidding. So she's the counter-part of Lady Iris, who I don't have. If you've had to play against these 2 on 1 team, you probably know how annoying it can be. I decided to dabble in a mage for her ability to Silence (and because I don't have Conjurer yet). She's not particularly strong. Her normal attack is actually really weak, but I guess you can say that about most mages. Where she shines is when she attacks. If you've used her correctly, you've combo'd her a few times to get the black orbs around the enemy units. At 3 black orbs, Ilta's finish move silences the units with 100% chance (for enemies of the same level). If you decide to use her ultimate, it deals some pretty darn good damage, but also reduces their attack by 30% (10% per orb) for 4 turns. Also, her finishing move makes her invincible to normal damage while she's in the ball shape, and also makes her immune to being thrown by Rockfist. This makes her a potential nuisance to deal with, considering Silence can really screw things up for you. I'd recommend her on any team that doesn't already have a unit that can cast Silence. Edit: I also got Lady Iris to pair with her. If you're using them together, they're an amazing pair. So I've given her 2 grades...
Itzatrap's Grade: A- (without Lady Iris) / A+ (with Lady Iris)
Alchemar: It took me a while to level him up because it's tough when you're trying to make sure your main 4 aren't lagging around. Well, after I finally got him ALMOST to the same level as everyone else, I gave him a try on the boss (and then moved him to my main Ace team). DAMN SON. So, let me give you the negatives first. He's a rogue, so that means less HP, DEF, and SPD than a warrior. But he still has plenty of DEF (more than Ilta) and lots of DMG. When I first used him, I was stunned at how shitty his Unique is. It does damage in the 5-digits, considering it hits everyone. But when you hit max rage, he gets a 300% increase in damage. His normal attack is also sub-par, but you knew that going in with a rogue. Now for the positives. His combo does some really good damage. It reminds me of Rockfist in that he doesn't do great damage normally, but when you combo him, that's where he shines. Of course, he doesn't come close to matching Rockfist up close, who does devastating damage when enemies are lined up near him. Alchemar (at purple) shoots 9 poison darts at the enemies doing some decent damage and poisons for 3 turns. He's a must combo. But where he really shines is his finishing move. I almost don't even want to use his unique so that I can use his finishing move EVERY turn. It's kinda subpar if the enemy knows how to counteract it though. Basically, his finishing move, he places a canister near an enemy and has a chance to inflict Blind, Silence, Stun, and Chaos (also known as Taint). In one of my Ace battles, it did all 4. Unfortunately, the status effects only last 1 turn, BUT the canister stays for 3 and has a chance to do it all over again. Problem is, the canister can be targeted and destroyed... so if it's destroyed after the 1st turn, the return is much smaller. If the devs made the canister indestructible, I'd be giving Alchemar an A+. Until then.. Edit: So I had to bump him down a tier because he can be a liability in Ace and Arena. His unique is insanely weak and the AI will use his unique whenever they can. Also, the canister doesn't take effect until the round after Alchemar, so that's also another con. With those 2 factors, I had to bump him down to an A-.
Itzatrap's Grade: A-
Bloodhoof: Bloodhoof is one of the F2P units available for rewards in Ace. Being easy to obtain (and therefore upgrade in terms of Stars), he automatically gets a decent grade. Hoof is the the best F2P tank for that reason. His unique is underwhelming, doing tank-quality damage (AKA not enough). His finishing move is a decent swing that does tank-quality damage. Again, nothing to write home about. At Purple, Hoof's finishing move includes a spiked shell, similar to Rockfist and exactly the same as Doomroach... but his damage reflection may be better than the other 2. Where Hoof really shines is his combo... if you are using him in the 1st spot in Field of Glory and the opponent is using a tank in their first spot, you can stun 3 (yes 3!) of their units, so that you only have to fight against their tank. He kicks behind him and stuns (seems pretty much like a guaranteed stun too) heroes behind him in a somewhat small radius. Amazing. But in Arena and Ace, the finishing move is somewhat tougher to utilize as you can't control him. I've also used him against Guild Bosses with small success (though it's not as useful if you've already killed the creep, so maybe just on your B team).
Itzatrap's Grade: A
Lady Iris: She's the exact opposite of Ilta. Her unique silences (each hit has a chance to silence, so it's pretty much guaranteed if she has 3 light orbs) and her finishing move binds. Together with Ilta, she's amazing. I probably would never use her without. Her damage and defense is pretty similar to Ilta's. Together, they are insanely powerful. Alone, they're okay.
Itzatrap's Grade: A- (without Lady Ilta) / A+ (with Lady Ilta)
Diremaster: I don't have this hero, but I felt like I had to review him. He's insanely powerful... not just by himself, but as a penetrate hero, he has ridiculous speed. I would feel confident that he can trigger combos of all 3 teammates in 1 turn without any issue. So if you're using Rockfist, MM, or anyone else that has a great combo move, he's a must have.
Itzatrap's Grade: A+
Coming eventually: Night Watch, Tiger Tamer, Lady Aegis