My most recent would be beating Elite Beat Agents' Jumpin Jack Flash (final song) on Hard Mode. Now there's just VERY Hard mode to go!
I also have some notable multiplayer moments, like in TF2 going straight into overtime, getting creamed, but then finally pushing the cart/taking the point whatever.
It's MOST satisfying when, in a capture map, the opposing defense just steamrolls your team straight back to spawn, setting up sentries and everything. Then, just ONE Spy or Scout gets past, and since no one on defense is back there, the dialog goes something like this.
"ALERT! The final control point is being-"
"WHAT?"
"Shit, someone get it! GET IT!"
"WHOEVER SPAWNS NEXT"
"How did they get past...?"
"YOU RETARDS"