Wings of Vi. I can't even beat the first area (though I refuse to not play it on the hardest difficulty soooo...)
Other than that, probably the "Remember Me?" custom level in VVVVVV (It's "Doing Things The Hard Way", but times way too much stuff. I don't think you ever get to a resting place and I know there aren't any checkpoints. And it takes a while just to fall up the thing on the side, like DANG that level is tall.)
EDIT: Aria in Crypt of the NecroDancer. Apparently Coda's even harder, how's that possible?