I think the hardest games would be (in my personal experience) these ones...
* Haunted Castle (was able to beat it in the easy version, emulating it and saving states... otherwise it seems impossible).
* Castlevania III (beat it, but spent a lot of nights doint it).
* The Adventure (because of the character and some very difficult passages of auto-scroll in-game).
* The Adventure: Rebirth in Hard Mode (I should mention it, since that's the way I've always played it; I find the normal mode too easy for a classic game, so I always choose hard).