I posted here but i still say aliens but for new reasons.
An alien can always facehug a predator and make better alien out of it.
I mean for all we know everything the alien face hugs makes a strong xenomorph version of the host. So one on one a predator against a pred alien is screwed, not unless the predator is a complete experienced bad ass that fought them before. So aliens are clearly Superior for they can survive anywhere and make strong enough soldiers anywhere to have the ability to control the planet. And by control i mean swarm and infest it.
They have evolved or been created so they are the perfect life form. intelligent, clever, sneaky, strong, fast, no sense of morality, able to adapt to almost anything, acid for blood, armored skin, able to climb on walls, and lastly able to take on traits of its host, to give it the best advantage on the planet or ship they are on. There's most likely a lot more reasons.
