Castlevania: Symphony of the Night and Castlevania: Aria of Sorrow.
I'm up to Dracula in the original game, and I simply can NOT beat that bastard. I always seem to die in his first form. I'm going to have to practice it a lot.
I'm kind of stuck in the catacombs of Castlevania 4. (The stage with all the spikes and spiders) It's kind of the first truly challenging stage in that game.
I am working my way through Harmony of Dissonance and Castlevania 2 as well.