If you has asked me this question a few weeks ago I would of defently picked Aliens but now I'm not so sure. I mean Aliens need humans and preds to eat and breed thus if they has their way they would kill all of them and they would go extinct, or the Aliens would be eliminated eventually anyway by the preds. I mean the pred always win, they won AVP1 and they won in AVP2 because the aliens were totally terminated and the preds only lost one ship and the hunter. Well I suppose you could say the Humans won AVP2.
Anyways if their was a war between Aliens and Preds the preds would win, they don't even want to exterminate the Aliens they see them as sport. The Aliens rely too much on the other species, if they evolved so they didn't need hosts then they would be the best but until then the preds rule.