I honestly like all of the levels. I can understand
Ivan 's criticism regarding Cainhurst, but I thought it had a pretty cool atmosphere - reminded me a lot of Painted World from DS1. I also really liked Nightmare Frontier a lot because it was so outlandish and beautiful in a very surreal fashion.
In terms of mechanics alone, discounting aesthetics/atmosphere, I would say that the beginning of Mensis' Nightmare Frontier was probably the weakest due to the frenzy mechanic forcing you to just run. (dunno if this was changed in the patches)
Favorite Fights: Gas Can, Gehrman, Amygdala (non-chalice), Nicolas Cage, Period Blood Queen
Worst Fights: Witches, One Reborn, edgelord katana faggot at the end of Eileen's questline (dunno if he was patched, but he was notorious at launch), that fucking giant dog in the defiled chalice
I recently replayed Dark Souls 1, and honestly the bosses in that game are a lot more impact imo. Too many of Bloodborne's play out the same way. In terms of bosses alone, I'd say it's something like Dark Souls > Dark Souls 2 DLC > Demon's Souls > Bloodborne = Dark Souls 2
On the note of DS2's DLCs, I've always felt that they were something like Sunken > Iron >>> Ivory. I didn't really care for Ivory to be honest.