Halo 2, Relic, I got in a Ghost seconds after the start of the match (FFA with some friends), turn around the corner and see two guys walking towards each other with swords. I hit the boost. DOUBLE KILL
Halo 3, 3p FFA on whatever it's called again, the standard map with the two bases and the river. I've got a Spartan Laser, aiming pretty much across 75% of the level at one guy, and JUST when it fires, the other guy comes down in a Banshee..... down to the ground..... right in my line of fire. DOUBLE KILL
Red Orchestra, leningrad (custom map, capitalisation [sic]), I joined the game in the late stages, 12 minutes to go while defending the last objective (I rarely play Axis anymore these days, and never have on that map). I'm a standard Strelok with an MN 91/30, crouching behind the sandbag wall at the end of the bridge, there's a machine-gunner on the barricade at the opposite end that I can't see except for the tracers he's firing. I shoot..... I kill. Excellent Marksman - Bronze
Halo 3, Forge. I splattered several people (in several games) with flying Scorpions. Barrels of fun.
Red Orchestra, any time I'm in an ISU-152 and one-shot a Panther. =D
Red Orchestra, can't remember the name of the map, Zhitomir 1941 I think, where you have to fight the Germans back in a winter landscape. I was the only tank driver, driving the available KV1S forward and ever forward, killing their Panzer III time and time again and suppressing their infantry advances (though most MG kills were bots, although I did kill one guy twice in a row once), basically single-handedly allowing the rest of the Russian team to advance and secure the map (that Panzer III on there can really bring the pain, if left unchecked it can pin down everyone across most of the width of the map). Not so much a single kill as much as a string of kills, but I'm pretty proud of it nonetheless. (Of course, it helped that they apparently had only one tank driver. One-on-one, KV1S > Panzer III.)
Red Orchestra again, some map in Stalingrad where you have to retake an industrial area with a trainyard. I was playing as a standard Strelok, and there was one vital passage towards the Assembly Hall building that was being camped by a couple of Germans with MP40s, killing anyone who came near. So after getting my nuts blown off twice, I wisen up and creep forward, lob a grenade into the doorway they were hiding in. Two kills in one. Of course, two more guys came out and killed me, and they were back pretty soon, but still it was a nice try (IMO).
Not a kill per sé, since it concerns a grand strategy game, but still. Hearts of Iron II, I'm playing as Germany and the only country I managed to convince to join the Axis is Hungary (and Austra ofc, but the Anschluss means that you basically annex it). So in 1938 I decide to invade Czechoslovakia. I position my panzer divisions to the north and south, hoping to cut the country in half and bypass the Sudetenland forts and sweep into Prague, while my infantry divisions keep the Czechoslovak armies busy in the Sudetenland. The campaign goes pretty much as planned, though it bogged down severely once I had the Prague area and had to help my clueless Hungarian AI ally take care of the tail, which played host to a continuous game of musical provinces, but once my panzer divisions arrived it was over pretty quickly. That the plan worked so well was in itself awesome.
Then, later, I learned about Fall Grün, an actual Nazi war plan for a 1938 invasion of Czechoslovakia, using panzer divisions to cut the nation in half and bypass the Sudetenland defences and head for Prague, while infantry divisions created a distraction keeping the Czechoslovak army occupied in the Sudeteland forts..... At first I was immensely creeped out, but then I realised that, despite the fact that whoever wrote the plan were Nazis, they were also generals, which means skill and experience. And I copied their plan almost to the letter without knowing.