Killing 11 people in a row as a heavy in TF2, in 2 fort. Dunno why it happened, i just charged foward and ennemies were just thowing themselves in front of me or something, pure carnage.
In SW-Battlefront 2; Jedi temple demolition level in coop (i think), i was a simple clone trooper on high difficulty, i managed to never die on that level while still fighting in the frontlines and taking on the jedi masters and somehow killed them all. (iirc my buddy was doing a great job at distracting them)
In several L4D games, when everything you do just works, when i just know that our team survived only because i made sure that nobody was left unprotected, and managed to dance with the tanks so they cant attack my teamates.