I have a few.
My favorite by far was on Red Orchestra. I was a Sniper, and had to cross a road to get a better view of the enemy. I take two steps out, and BAM! Tank, right in front of me. I jump, fire (it's a reflex thing) and KILL THE TANK COMMANDER! He was just ducking himself back into the tank, and I pegged him in the head. It was hilarious.
My second comes from Metal Gear Online, and is not so much one great kill as it is an improbably long string of mostly-cool kills.
I was doing my usual 'anti-sniper' routine (I was using an Assault Rifle), running from sniper-nest to sniper-nest, sniping other snipers (sniping!), when I run headlong into not one, not even two, but three dudes with knives. So I do what I always do in this situation; switch to my knife, try to knock one over, and RUN THE HELL AWAY. The one I knock over falls unconscious and falls of the building, the second tries and fails to grab me, so I tackle him, whip out my AR and kill the last, turn to kill the tackled dude, and, just to have some fun, drop a grenade on the now-unconscious first guy. That grenade ended up killing two enemies, because he had just climbed the ladder and promptly asploded, falling off the building to his hilariously-well-timed death.
And that's it... I had a third, but it escapes me at the moment. Stupid thoughts, slippery little devils.