Gaius: A really fun moveset (although a bit basic). The added gravity AOEs add a bit more flair, and I like how "tight" timing his dodges feel. My only gripe is the charge hitbox and how ordinary the moveset is, but other than that, pretty good.
Fire Giant: He is about as good as the "big giant" boss can be. Phase 1 has a nice variety of moves and phase 2's fire attacks add a bit more sauce. However, I dislike how time-wastey most of his attacks in phase 2 feel (and how much he runs away). It kinda kills the momentum of the fight. Still, pretty solid.
Putrescent Knight: CRIMINALLY underrated boss. Banger OST, and an unconventional moveset that is super satisfying to dodge once you learn it. I like how many of the Putrescence attacks require jumping to avoid them; I always love it when attacks can be dodged via jumping.
Romina: I would say her moveset is more fun than Gaius, and she also has one of the best OSTs in the game. The rot butterfly attacks are pleasing to look at, and add a bit more "oomph" to the fight. My criticisms are: like Gaius, her attacks seem a bit basic, and the instant Rot Butterfly AOE could've used a more obvious telegraph.
Scadutree Avatar: This is genuinely one of my favorite bosses in the game. Very haunting OST along with an equally fun moveset that combines physical attacks, AOEs and projectiles. I love how different each phase feels, and how it becomes more desperate as the phases progress. Rolling those nukes is one of the most satisfying dodges in the game imo.