I don't remember what it's called but the one for killing the top player on the enemy team 10 times. And 1000 kills with a stealth bomber. I don't think that I've ever seen that one displayed even though somebody else has bound to have gotten it.
The one's I liked to show off the most were 1000 headshots with MWR, 1000 headshots with Barrett .50 cal, and 1000 kills with a Pave Low. I think the hardest one to get though is 1000 headshots with the Five Seven. Nobody will EVER get that legit.