This is a great thread. I don't have any real major accomplishments. I killed the Lich King in World of Warcraft with a group of 9 other scrubs. That probably isn't very impressive, but I think it was the only current raid I ever completed.
Call of Duty: Modern Warfare - I had a round with like 70 kills and only 1 death.
I created an alternate soldier for Battlefield 2142 called Yougotsliced and all I did was try to kill people with the knife (and collect their dogtags). I had it around a 14:1 kill/death ratio with knife.
I sunk 8 out 12 ships in a World of Warships match, once.
But probably my favorite accomplishment was making a coach for NCAA football dynasty mode, getting hired by t.u. and running that program straight into the ground. They fired me after one season, going 0-12.