Seriously, if you want good challenge, play all the old Castlevanias (NES, SNES, GB). They don't have that RPG element where you level up from the PS1, GBA and DS... which obviously makes the game a lot easier. Stay away from the 3D ones, please!
Like everyone said, the series' best game is Symphony of the Night for the PS1, but you should really try Harmony Of Dissonance on the GBA. I really enjoyed it, and the main castle theme is just catchy! The Sega Genesis one is also very good.
For your question, I'd say:
Castlevania > Castlevania III > Castlevania II.
Have fun!
Random game thought :
Why is Bionic Commando Rearmed 2 getting so much hate? We finally get a real game and they're not even satisfied... I'm starting to hate the gaming community so f****** much...
