Favs (please don't make me choose): Orphan, Maria, Ludwig
These boss fights are so very exciting and demanding, and beating them is always satisfying. They each have their own feel, Maria feels like a duel, Ludwig feels like a desperate fight against a huge monster, and Orphan feels like a parry party until the second phase, which is a dodge-and-pray fest.
Bosses that I hate are Rom and Micolash, because whenever they kill me it always feels like BS, whether it is one shot uninterruptible magic or homing space potatoes, it always sucks.
My least favorite boss has got to be Laurence though, it always feels like a grind where you are either healing or mashing R1. It just sucks and I only did it for the trophy.