I’m pretty desensitized to horror movies now, but I saw Forbidden Planet, in a theater, when I was a kid, and parts of it scared the hell out of me. There’s the scene where all you see are the giant sloth’s footprints, then the ladder bending, then the screams from inside the spaceship. Later on, when you see the outline of the monster in the particle beams, it scared me too.
The finale, where the monster is burning its way through the blast doors, did me in. It’s the only movie that ever gave me nightmares. Watching the movie now, I still get a chill. And I marvel at how good the SFX were, considering when this movie was made.