#5: Trevor from Curse of Darkness.
Mainly the setup for this one. Trevor threatens Hector, badass music plays, and decent fight ensues. Not hard, but fun.
#4: Galamoth from Symphony of the Night.
This fight is difficult enough with Alucard, and is refreshing after some easier bosses in the Inverted Castle. Beating him with Richter is even harder. No Hydro Spam, here.
#3: Soma from Dawn of Sorrow.
Again, setup here is nice. I usually fight him with only Julius, which makes him pretty hard. Alucard and Yoko have an easier time with him, but it's the small space that really makes him even remotely challenging. Good and fun encounter.
#2: Death from Castlevania.
This fight is still my least favorite in Castlevania history. He's so tough that I still end up cheesing him with Holy Water. This battle is still why Death is considered a menace, even if his later incarnations fluctuate in difficulty.
#1: Dracula from Dracula X Chronicles.
Holy smacks. Third form is a beast! Form one and two are predictable, but form three keeps you guessing and memorizing! The developers knew this, too, because the music is a beatiful extension of Dance of Illusions, and Dracula seems to always say something different before sprouting his wings. It took me forever to finally beat him, because he always threw out something new, but I never got frustrated. This fight inspired determination, and that is why it is number one!