52 Win streak in Lightsaber duel in Jedi Academy
Or that time in Medal of Honor, on a custom map (forgot the name). The match always starts with nade spam to eachothers base, and bad luck made me end up on my own, 10 seconds into the match, with my sniper rifle, against 7 krauts at close range, still at the spawnpoint, where there's only 2 possible ways to run.
Guerilla warfare ftw, stealing their guns so I could confuse them when they heard gunfire, switching to pistol quickly instead of reloading (CoD4 is the first game ever to actually recommend weapon swapping if reloading takes too long, I think... I'm pretty proud I already knew of the tactic 5 years prior to that).
Anyhoo. 5 minutes and 7 dead Germans later, I got asked to join clans by half of the server population ^^
Another MoH one... I was on another clan's server, there were 28 people, of which 27 from that one clan, me being the outsider (but well befriended with them, they were constantly asking me to join). I was number 1 in the ranking, 2 maps in a row. Wewt.
A bit more recently... first time I played CoD4 was at a lanparty. I was still configuring the controls when someone asked me to be his partner at a competition, because he couldn't find anyone else. I accepted, and could only mess around for about 10 minutes in MP before the actual compo started. So basically I was level 3, facing nothing but level 55's. We stranded in the finals, because the other team had grenade launchers (which I did not have), and just kept killing us from afar with them. Really lamely played, but meh.
At the same lanparty, played some UT2k4, 8 man FFA. I went shieldgun-only, and won. Always hilarious when you see 5 people in the same room trying to kill eachother, you barge in with a charged shieldgun, and they all try to get away from you asap. Needless to say, they didn't succeed in escaping me that well.