5. The Thing
Appears in: The Thing (1982)
Scariest moment: When Copper (Richard Dysart) tries to resuscitate Norris (Charles Hallahan) with a defibrilator, Norris' chest caves in, giving way to massive fangs that bite Copper's arms off at the elbows. Seconds later, a large, oozing, tarantula-like creature with Norris' head jumps out of the open chest.
Weakness: Sausage parties in the Antarctic wilderness.
The scariest thing about the parasitic alien in John Carpenter's The Thing isn't its appearance, since, technically, it changes appearances numerous times. Which leads us to the reason why it's so damn effective: The Thing can be anyone or anything.
Attaching itself to various characters throughout the film, the devious life-form turns the humans against one another through simple implication, leading everyone to suspect their neighbor of being inhuman. And whenever it physically manifests itself, the Thing turns its hosts into everything from tentacled spiders to indescribably grotesque monstrosities.