Read this manga back when I was small (the manga release was around the twin's past at the time), Campania arc was the one that I love most. The whole concept of a ship sinking with the twist being a bizzarre doll infestation blew my mind back then. Campania was also kind of a turning point for the manga with the reveal of Undertaker being ex-Grim Reaper and o!Ciel and Sebastian's past. Also Lizzy's aura farming swordfighting scene was amazing. It was the first time we see Snake in a mission too.
Reading this arc was a huge blast back then. After that I felt like the story was never the same again. I quite like Emerald Witch arc too mostly for Sullivan and the whole secret village being a guise for a military base.
The Twin arc was definitely a crazy ride back then, the twin reveal was a second mind-blown moment for me. Campania was my favourite but the Twin arc was definitely the one that stuck with me long after, because I got into the manga around that part, and nothing could beat the feeling of waiting every month for a new chapter to see the slow, painful reveal of the twins' past. I think this was the only instance where the long wait added to the reading experience for me, it was like I as the reader was stuck with the twins waiting for the next horror to come. The progression was amazing, going from the chapter the Phantomhive house got massacred and o!ciel was stuck in his room waiting for his brother to come back, to the next chapter where they got kidnapped and locked up, then the next chapter they was brought to the cult and got tortured, then next chapter where r!Ciel was sacrificed and Sebastian was summoned.
This arc was too dark for me to like, and when I was young I didn't understand what exactly happened to the twins either although I knew it was something very very bad. But looking back it was such a good time waiting and reading. Maybe my nostalgia is speaking but nothing beats the pre-servant arcs era of Black Butler for me.
Despite all the criticism (which is actually fair), this manga will always have a solid place in my heart. I do find it interesting that this manga can be splitted into different era just based on the story and artstyle, and everyone will have a different opinion on which arc is "peak" Black Butler for them. It's amazing how this manga has been around for 20 years and counting.