Dracula X for the SNES definitely - I really enjoyed that game when I played it for the first time recently, after having played Rondo of Blood, and frankly I enjoyed it more than I did RoD (I also enjoyed RoD). I've been playing through all the Castlevania games in 'chronological' order, putting all games on the timeline regardless of canonicity, and it's the Classicvania's that I've been enjoying the most and Dracula X (Vampire's Kiss) is basically the last gasp for the Classicvania's until I hit Bloodlines.
I also didn't mind Castlevania: The Adventure, but Belmont's Revenge is just so much better a game. I was hoping I would have liked Legends more, because I like Sonia and her being written out of history just raises her profile in my eyes, but I think I enjoyed Legends the least out of the three GBVania's - it's levels just seemed to drag on too much.
Just finished Circle of the Moon and that game was great! Haven't reached the N64 games yet but am hoping that they will be underdogs that I can get behind as well.