First up: Alien
It spits acid. It's has two mouths. It's got a giant tale that can take out a truck. It climbs faster, jumps higher and destroys more effectively than any other creature you might encounter in space. And the worst part of all? It uses you as an incubator. If that doesn't add insult to injury then I don't know what does.
No other space creature before or since has been as scary as the alien in the Alien movies. Not even Predator. (Alien vs. Predator? As if there was ever even a contest? Yeah, right.)