Well predators & humans don't eat each other, lol, so I guess it would have to be the aliens. But as DD stated, they are all from completely different planets so there isn't really much of a food chain between them
Although if by "food chain" u mean which is the strongest & most dominate overall then I would have to say the preds. Aliens are naturally born survivors & killers, & are, in essence, at the top of the "food chain" but the pred's exceptional intelligence has helped them forge weapons powerful enough to obliterate entire planets. If the predators weren't all about honour & the hunt, & just plain wanted to wipe out the aliens (or the humans), I'm sure they could do it with relative ease.