If I had to choose....
Rogue Warriors: FFA Deathmatch (8-12 players)
Species War: Alien, Predator, Marine Team Deathmatch (4P vs 6M vs 8A)
Natural Selection: Team Deathmatch - AvP, AvM, AvA, PvM, PvP, MvM (6v6-9v9)
Impregnation: A pack of Aliens must impregnate as many civilians as possible, with a team of marines trying to defend them. Maybe have it last a couple rounds, and one life per round. (10A vs 8M)
Hunt: Similar to "Impregnation", except a pack of Predators must get as many trophy kills as possible on humans, while a team of Marines try to defend them. (6P vs 12M)
Slaughter: Basically 6v6v6, except you add in about 10 or so A.I. controller allies per team. So we're talking 16v16v16 on maybe select maps that are built only for this. Would be intense!
Survival: 4 Player Co-op (Already announced, but I am hoping like in CoD WoW every 5-6 waves, a group of Predators hunt you...starting off pretty basic and easy, to later challenging, invisible, and tech ready)
So yeah, what do you guys think?