The most insane death I've had, me and a friend, who has a way of always getting good at games really fast, was playing 1v1 on Valhalla in Halo 3 right after launch. After a while, I was no longer able to tell where he was, and knowing him like I do, I went out into the open, to cover as many angles as possible. I kept looking behind me, left, right, just spinning, trying to get a glimpse of him. And all of a sudden, he assassinated me. I am still unable to tell how he pulled it off, 3 years later.
Me most epic kill would have to be in Frontlines Fuel of War, where we were playing on ths assymetrical night-map where the attackers started at a bas covered by a mountain top, with a bunch of helicopters, while the defenders were dug into the small town-base.
One of the points of capture was a generator that had to be destroyed, either by weapons or placing a explosive charge on it.
As I was flying(alone in the chopper) towards it and shooting at the generator, I suddenly saw a big flash, and a smoke trail coming right towards me. Realizing that I've got a homing rocket to deal with, I quickly jump out of the chopper, which is blown into a fireball of debris and hull the moment I leave it, and deploy my chute, watching the momentum of my chopper carry it towards the generator, hitting it, blowing it up, and killing 3 defenders.
So awesome.