My most memorable killstreak was on MW2, Kerachi. Rushed the sniper area above the store room and got 4 sniper kills off the bat. Claymore took out the first guy coming up the stairs, AA-12 shotgun got the next two, threw up all my killstreak rewards (UAV, Care Package, Harriers), UAV let me get the drop on two more guys, killing the second with my very last shotgun shell.
Realizing that thanks to the 4 Harrier kills I'm 13-0 in under a minute, I ignore the ammo drop, run to the carpark and hide for the rest of the match. Cowardly, but I typically don't get more than a 7-streak, let alone right off the bat so this was a unique opportunity. Fortunately I had Cold-blooded and Ninja on, and no-one really goes back there, so i was able to get the "The Show" and "Supernatural" accolades for the first (and only) time.