2. Alien (1979)
One of the best horror movies ever made while also one of the best Sci-Fi's ever made. Alien presents an incredibly realistic portrait of the future by having all of the characters just be regular people. The universe that Ridley Scott directs also eschews the glamorized version of space presented in Star Wars and Star Trek which gives the scares a sense of urgency because you know that the majority of the characters aren't going to make it out alive but you still feel as though they have a chance. The Xenomorph is also the greatest movie monster ever conceived because it is used in an intelligent way to add a deep layer of symbolism to Alien. The constant evolution of the beast is also incredibly frightening because you can never know for sure what it will look like next and what it is capable of.