Aliens, hands down. They don't need technology, they don't look even remotely human. They don't attempt to communicate with or join sides with humans. They are inherently vicious and uncompromising monsters. Tooth, claw, and a razors tail, that's all you get. There are no human emotions like honor, merely an unceasing drive to consume every living thing and spread the species. A lone predator is an isolated problem, a lone alien is the beginning of an extinction level event. Drop them in any story, on any world, and they will take over. They're simply unstoppable. And what they do, and how they do it, is utterly horrifying. They are, as it has been best said, "A Perfect Organism".
