Hmm hardest level. In SRB2 i would say Silver Cavern from the Mystic Realm. Curse you Mystic for making those infernal slippery platforms >_<
Ikaruga fourth level, just holy crackers thats crazy! (actully i can do the rest of them apart from chapter 5 quite well <_<)
And of course, any water level in any game ever that requires you to get air somehow.