Alien
For a movie released in 1979, this genuinely feels more intense and effective than a lot of modern horror movies. The pacing is slow in the best way possible it gives you just enough time to get comfortable before filling you with absolute dread. What surprised me most is how beautiful the movie looks. The production design, the atmosphere, the claustrophobic feeling of being trapped in space everything still holds up insanely well. Space has never felt this terrifying.
And Ripley? An absolute icon. Watching her gradually take charge while everything falls apart around her was so satisfying.