4 kills with a single grenade in Snow Valley tunnel. Normally that would be funny but the fact they all stood together and watched me throw the grenade at them and they didn't even kill me was the best part.
Funniest kill(s) I've ever made would be shooting other peoples mines when my teammates walk up to them. I would wait for my teammate to walk up to them, then shoot them so they blow up. It's hilarious and your teammates and the person who got the kill both get a laugh out of it.