I would say that my personal favorite is Castlevania III, but the Japanese version: Akumajo Densetsu. It has an additional soundchip embedded onto the cartridge, allowing some kick-ass basslines in the soundtrack, which the PAL or US version doesn't have.
I'm telling you, if you're a die-hard Castlevania fan, I recommend picking up a Famicom just to play a copy of that game alone.
There's a magic like no other playing Rondo. This is going to sound super crazy