vamprie killer, haunted castle, CV III and CV adventure are the most difficult ones I believe.
Can I play Haunted Castle on an emu? Isn't it an old arcade game, and if so, do I just use mame?
I lack a Wii (wait...that doesn't sound right), so I can't play CVA. I've beaten the original CVA on the GB, so I figure that pretty much covers it. If I ever get a Wii

, I will definitely pick it up though.
In Reply To #7,
LOL
you do make a good point; it's all about the reflexes, son
Naming yourself after a Belmont you have yet to master is shaming that character....
....alright I'm just kidding. Don't take anything I'm saying seriously
Richter. Except for the reflexes thing. Work on that. Play as Maria if you want to beat Rondo, she basically puts the game on vadge difficulty.
If you want to go it as Richter, try looking up a FAQ on gamefaqs.com, you should be able to find a FAQ with will list the easiest route (some paths are much more difficult than others). Other than that, memorize patterns, especially for bosses. When a boss is about to do a any particular move, they will do what can be referred to as a
tell (like in poker). Each move has it's own tell (usually). Through memorizing these patterns, you learn when to attack and when to GTFO of the way. Sorry if that sounds condescending, I've had friends that have been helped by that advice because they weren't properly reacting to what the boss was doing.
After heavy training, now I can complete Rondo of Blood from beginning to the end - without using continues! 
And a lot of other Castlevania games. However, Castlevania I is still a "mission impossible" to me. 
CV1 is probably the most difficult for me, as it's timing can be extremely unforgiving. I've actually beaten the Reaper w/o holy water, but after doing it once I was satisfied.