The obvious one would be IWBTG, getting past/to the first boss is an achivement for me and most other people, but hte game can be worked though, and I agree with the people who are saying games like IWBTG and Super Meat Boy aren't really hard, they're just tiresome.
For a hard game, Metroid Prime Hunters for the Nintendo DS is the most frustrating game I've played - easy enough shooting things when cover's around, but when it isn't, and the control system doesn't like left handers (like me </3), it makes the game almost impossible - never got past the first room - let alone the first boss.