I have 2 from Halo 3.
The first one was on Snowbound, at the Shotgun spawn point. I had noticed before that the area was surrounded by explosive containers and decided to try and lure someone into said hallway and kill them by blowing up the containers around them. Needless to say, it worked. Twice.
The second one was earlier in my career with the game. This was more what happened afterward in the saved film. The game itself was a game of Team Slayer on Valhalla. In the course of the game, I think I got, like, 3 sniper kills on people going off the man cannon. Me and my party went into the saved film later on to check out the game, and I was waiting to check out the amazing kills I made. I paused at each one of them, and I think it was the... second one, but at the time I paused it, the guy was still going through the air and hadn't gone into "dead mode" yet, and the sniper trail remained, as well as the blood from his torso, and just the way it was all stopped made it look like I had just sniped him in the crotch.
In fact, here's a mock poster I made with a screenshot from that game: http://bp1.blogger.com/_ct_-0L0_BII/Ry_Q8x59w5I/AAAAAAAAAoo/_LOw3XCdbF0/s1600-h/eunuchtacular.jpg
EDIT: Also, any time I've ever thrown a knife into a guard's back in Assassin's Creed, only to have him fall off the roof of the building he was patrolling and landing in the streets below. The sound of people freaking out cracks me up every time.