hm, out of the classic-style Castlevanias, I think Castlevania II: Belmont's Revenge on the GameBoy is probably my favorite. Nostalgia may probably play some part in this, since one of my brothers and I grew up with this game (as well as Castlevania 1 and 3 on the NES), but even when I replayed it for the first time in ages on the Castlevania Anniversary Collection, I had a lot of fun with it! It's a really well-made platformer, imo!
Also, it's got some of the best chiptune music on the GameBoy, hands down. the 'Crystal Castle' theme, for instance, is still one of my most favorite chiptune tracks ever, tbh!