Myself it's symphony of the night, for multiple reasons. Including the fact that it proved that 2D platformers aren't dead.
I really liked the 3D games, not sure why. Gabriel and Leon are my favorite Belmonts
I loved Portrait of Ruin, but mostly because of the two main characters, obviously.
Despite the flaws, I really enjoyed Curse of Darkness. The best one, in my opinion, is Aria of Sorrow.
What do you think?