The Day the Earth Stood Still (1951)

The serious one to start with. An alien visitor and his robot, Gort, arrive with a warning about humanity's weapons, and Robert Wise wanted it to look as realistic and believable as possible, which makes the message land harder. Bernard Herrmann's score uses two theremins, and that eerie wail became the sound of 1950s science fiction. Learn the phrase "Klaatu barada nikto" before you press play; the film never translates it. It joined the National Film Registry in 1995.