Which game you thought was the most difficult?

Try SC2:Hots mp with noob team. It's killing me, I swear!

Castlevania on the original Game Boy.
Haven't played the first one, Ghosts 'n Goblins, Spelunky, Prinny, enough of Super Meat Boy (don't own it, played at a friend's house) or any of those "masochistic unfair games" like I Wanna Be The Guy (maybe except a bit of Syobon Action) so I don't have that much of experience with these ultra hardcore games.

Quake against a really good player counts as well?