It's hard to pick just one. Aria of Sorrow, Order of Ecclessia(especially with the non-linear hack) and Symphony of the Night are probably my favourites of the igavania style ones. Bloodlines, Rondo of Blood and the original Castlevania are probably my favourite of the classic style games.
I also have a soft spot for Castlevania 64 but that could just be because it was my first Castlevania game and I didn't even know it was a series when i played it other than my dad's vague ramblings about some Vic20 text based Dracula game he'd played that he was positive was the original Castlevania game. We'd never owned any of the nes games and I'm still not sure if he ever actually played any other real Castlevania game, one of his friends also had an nes, but I did eventually figure out what Dracula game he was talking about and it was in fact totally unrelated to Castlevania.