The hardest thing I completed (not the hardest thing I attempted) was beating the Aztec level of Goldeneye on 007 difficulty; with AIdamage 200%, AIHealth 200%, AIAccuracy 100%, and AIReactionSpeed 200%. I fell victim of a April Fools prank made by a magazine I have, EGM, that fooled readers that by completing the level at those settings, they would gain a secret cheat to gain more characters in multiplayer. I spent weeks trying to beat the level. When I finally did, and didn\'t get the cheat, I was severly pissed. I didn\'t find out that it was a prank until the next issue.
F-Zero X is pretty hard too.