I recently picked up CVIII to find that it is pretty hardcore. (Didn't use save states, walkthroughs or cheats) I didn't find CVI was as bad but have heard others disagree. Which CV games would you rank as the top three hardest?

I thought these Castlevania Haikus were pretty appropriate:
http://thedarkopera.weebly.com/castlevania-iii-haiku.html

Well, II does have some non-linear aspects, and it's much easier to play it because you aren't forced to kill most enemies in the game.  I always found the first one to be the most difficult, it has some really challenging sections in later levels and the Count is quite a tough boss when  you first play him.  Even after getting the pattern down it can be difficult, but if you learn to use the holy water properly it's pretty easy.  Still takes practice!