Pretty much any sniper kill. I'm great at FPSs and as a sniper I am just God. I don't mean that as a boast either, I literally am just amazing as a sniper. Not so good with most weapons, but put me in for the long wait and long range action and I will rule the battlefield.
So yeah. Anyway, I'm also pretty good in dogfights in vehicle combat, and on Halo Custom Edition I downloaded one of the more popular maps, Coldsnap. 36km square in game, consisting of a wide open snow covered plain with two bases in mountains at either end (where you spawn, but you can't enter them when you leave). A giant crevasse across the battlefield with a single bridge, and the entire battlefield is contained by mountains on three sides and a sheer drop on the fourth.
In this map, there are air vehicles that were custom made by the developer. Jets, which are simply fast attack aircraft like the Eurofighter Typhoon, and Longswords (the same ones that you use to escape Halo in the original game), which are slow but have tons of missiles and a nuke that can be dropped from underneath the aircraft. So, I was in a Jet in a CTF game, flying directly to the flag point. My plan was to use the Jet to sneak up by flying in the crevasse where no-one could see, then come up right by the flag, hop out, grab it and get back in the Jet. Same method for extraction.
I come out of the crevasse to find a sniper shooting at me from a cliff near the flag. The guy had actually managed to get up onto the mountains and was firing from a hidden spot, no-one else knew where. I had literally my entire team of me and seven other people in Jets and Longswords trying to find him, since there was no other way we could get the flag. They all died. I ended up flying around the mountains with two enemy Jets on my tail, before killing them both in a dogfight inside the crevasse and then coming back and killing the sniper. The only thing cooler was that I managed to get the flag after all that and effectively single-handedly won that game. I've never before or since been able to recreate that either, which is a shame, since it was some pretty epic flying I did that day.