Dracula's Curse. It was one of the best games I'd ever played back in 1991 and it still makes me extremely nostalgic whenever I play it now. Playing the Famicom version a few years back was a new experience and was like playing for the first time again - when I play that, the enjoyment I get from it isn't even something old enough to call "nostalgia" since it's still relatively new to me, and it rivals Super Metroid as my favorite game of all time.
Bloodlines. To me, some subtle element of the artwork, the style of the music, and the "feel" of the game harkens back to the first Castlevania, making this one quite possibly my favorite ClassicVania to date. It's still a joy to play and it still rubs me the wrong way (like it did in 1994) when people compare it to Castlevania 4 and dub it "sub-par" by comparison.
Belmont's Revenge. It's everything that Adventure should have been and wanted to be and is as good a ClassicVania as any.
Simon's Quest. I played the first game first and played 3 extensively before I'd played much of SQ, but something about SQ's "feel" defines Castlevania beautifully for me. I have a lot of nostalgia for grinding for hearts in a few specific spots, finding the Fire Whip for the first time, "cheating" and playing the later half of the game first (with everything taking 16 or 32 hits), and how desolate and empty I felt going through that gray, empty town knowing that it was the last in the game. Entering Castlevania and hearing the music start up with the visuals still hasn't lost its magic for me. It's a beautiful game.
Dawn of Sorrow. There were a lot of "THIS is Castlevania" moments the first time I played it: walking around in the Alchemy Lab with "Dracula's Tears" playing in all its jazzy glory; the vast Chapel with its Castlevania 3-esque stained glass windows and Castlevania 4-throwback dancers; the Ruins, complete with "Vampire Killer" and phantom bat; the Condemned Tower; even the Abyss screamed "this is how Castlevania is supposed to feel" with its terror-inducing lava background and almost incomprehensible music. I had doubts here and there with the anime-styled character portraits, silly plot, questionable boss battles, and a few "not very Castlevania" songs, but every time I've put it down for months and come back to it, it's always brought me the same joy as that first playthrough. It's probably my favorite MetroidVania, though I realize SotN does it better.