Why not that one gamemode where a set team of like 3 people chucked random items down a chute that made everything fling at about a million miles per hour, at another team who has to dodge the items?
Or the one where there's a select few snipers inside of a floating nest, and the rest are racers that have to make it through a floating, semi-hard, extremely long course?
I can remember the Halo 3 days when I'd just join seemingly random, full lobbies and do these random custom game sessions. Halo: Reach, too. Not in any other Halos, though.