Halo Reach: Waterfront on firefight. Those snipers always seem to spawn when you're desperately jetpacking away from an Elite swordsman with a few notches of health left. BZZZZZZZZRT. Goodbye, perfection medal. Oh, and any map on Elite Slayer, since it's made up of all the weapons you ignored in campaign and never took the time to be any good with.
Black Ops: Firing Range. It's a fun map, but I'm getting awful tired of playing it two times in a row, ten times a day.
Bad Company 2: Panama Canal. There are three kinds of people on Panama: those that get tanks, those that stay put, and those that get shot up by campers and tanks. I'm also not crazy about Heavy Metal, since, let's face it, the entire match hinges on who captures the AA gun in the middle, and if your teammates refuse to help you get control of it because they'd rather run up the valley walls and snipe at distances they'll always miss from, it's game over. Oh, and Atacama on any mode, because I know I'm in store for a shitload of walking.