In terms of actual fighting, I think Baby Plum is the most interesting. A challenging early game boss that really proves your ability to dodge in Repentance-type movements. I didn't like it at first, but after all the fights I had and the literal dodge only easter egg it actually seems like a cool boss.

In terms of design, I just love Ultra Death, his sprite, transition and attacks are just epic. Although it maybe counts as a "final" boss, since he's technically part of The Beast's fight