A large arena surrounded by multilevel ledges. Lots of weapons, lots of ammo, and lots and lots of bobs... smaller groups will have trouble staying alive long enough to kill other humans, because the bobs will gang up on you. Larger groups should be okay, though...
An open shallow pool with several simple corridors around it. Hallway space is tight, so if you're out of ammo, you're dead. Carnage rates should be high, especially with aliens on...
It doesn't get much simpler than this. A big figure-8 racetrack, with lots of baddies, and lots of stuff to kill 'em. (And your friends, of course.) All starting positions are in a row, so you could play this one as a real race... first one across the finish line wins. (Mario Andretti never had to worry about being shotgunned from behind...)