Oh I've got a lot of these:
1) Beating Gears of War 1 & 2 on insane by myself.
2) Beating Modern Warfare on Veteran.
3) Completing every single Splinter Cell: Chaos Theory mission on 100% (no kills, never noticed, etc).
4) Beyond Good & Evil's final boss fight. I GOT YOU, YOU KEYBOARD FLIPPING SON OF A *****.
5) Beating Earthworm Jim. Jesus F*CK that game was hard.
6) Beating Torin's Passage (long time ago, great game)
7) Coming in 1st place on the Heaven/Hell and Nightmare maps in Jet Moto 2
8) Beating the final boss in Warhammer 40k: Dawn of War
9) Playing Mass Effect 2 in 3 days and feeling the exhilaration when I finished.