8. Alien (1979)
Never before had sci-fi and horror been mixed to such perfect effect. Horror and sci-fi had been united earlier with films such as Invasion of the Body Snatchers and Them!, but it was Alien that took the peanut butter and chocolate-like genre blending of sci-fi and horror and proved it could be something more than a cheesy B-movie. Alien was truly terrifying without being campy, and you can see its influence in modern movies such as Prometheus (obviously) as well as other horror classics such as Predator and The Thing. The Xenomorph is the most terrifying movie monster ever put on the screen, since it is really the perfect killer. It bleeds acid and its only goal is to reproduce or kill everything. Before Alien there had never been a monster anywhere near as frightening as the Xenomorph, and after Alien, many tried and mostly failed.