I've played 1, 2, 3 and 4 using emulators and also dracula x. I liked 1 and four better as well as Dracula x. I found Simon's quest way cryptic.
I really like Castlevania 1 and Aria of Sorrow. Potrait of Ruin is pretty solid as well, but I never finished it.
Lords of Shadow had probably the best story and top quality gameplay; as well as voice acting, music and combat.
Harmony of Despair is fun when you have a few other friends who are beasts and can run through the level quickly as form tactics.
SoTN has some of the best gameplay due it's MetroidVania style (which are rare).
I have to say it's either Aria of Sorrow or Harmony of Dissonance.