My dad once asked me to play Rainbow Six: Rogue Spear against some of his work friends.
He gave my character a shotgun. Loaded with rubber shells. (It was a joke or something, I suppose.)
I defeated them all single-handedly, without even knowing I used the game's absolutely most useless weapon that required a full magazine to the head to kill someone in a game where one or two shots downs you, on average.
IIRC, his friends attributed it to genes. Apparently, because my dad's been a desk job lieutenant in the swedish army for some long amount of years, I'm a 10 year old FPS prodigy.
Right, whatever.
Still, that has to be the one. Odds extremely stacked against me, I, alone, defeated a bunch of military guys in a military simulator game using the weakest weapon imaginable.
I sadly have no deep memory of this, but the legacy of my heroic deed of pwn lives on in the Swedish Army to this day.
Sub-moments would include...
Double-jumping in HL-2, using a rocket fired from an enemy rocket launcher (I swear this happened. I don't know how, but he fired a rocket at me in a tight corridor, I jumped and jumped AGAIN off of the rocket, or so it sure seemed, and killed him with my handgun.) This was in LAN. Many yells of profanity directed at me ensued.
Resistance: Fall of Man. Slaughtering a whole bunch of people in survival mode, getting almost 50% of the map's collective kill count, without dying once. (In R:FoM, my performance varies between "GODLIKE!" and "newbie", very very fluctuating.)
LAN game of Soldat with classmates, went from one end of the map to the other, killing every single person in a glorious dance of death involving every weapon at my disposal (shotgun, knife, grenades), with only my helmet getting shot off as for my injuries. Worthy of mention is that I did not stop moving in the exact same direction even once during it.
There were a lot of cries of profanity, directed at me. And the shotgun was banned from my use.
Me and a friend used to have a relatively long-lasting (but very friendly) rivalry in SSB:M, I was a Shiek user through and through, he used Dr.Mario. We were always perfectly tied. One day, we decided. Ninety Nine lives. No items. Final Destination map. This was it. This is where we decide who is the true champion of our epic SSB:M rivalry.
Blow by blow, we eat each other's lives away until we have one left each. And my damage is approaching 200%, while he's fresh as a daisy. I get blown off the platform, almost get back, but he jumps out to hit me into the depths below. I teleport, using my up-B as my last possible action... He meteor-smashed empty air spectacularly, failed to up-B back to the edge, and plummeted to his death, me following him a few meters above.
Victory was mine.
And that's it, as far as I can think of right now at least.