The Thing (1982)

The science in The Thing is dubious, but I don't care, because this is one of the most entertaining films ever made. A research team in Antarctica discovers an alien that can perfectly copy any living thing, and nobody can trust anyone. How exactly does it copy a whole person, memories and all? No idea. But I love the blood-test scene, because it's basically an experiment: come up with a hypothesis, design a test, and see what happens (very tensely!). The practical effects are grotesque and brilliant, and the ending is perfectly ambiguous. Pure paranoia, and nearly perfect.