Alien

Though it spawned several action-packed sequels (those face-hugging aliens really can breed), what makes the original, Ridley Scott-directed film effective is the atmosphere of claustrophobic horror. It's true that in space, no one can hear you scream; there's also nowhere to run. Suffocating blackness outside, a huge, acidic-blooded monster inside ... these are not good odds. Watching Sigourney Weaver's Ripley best them, even as the rest of her crew is decimated, never ceases to thrill, terrify, and make one eye the starry sky just a little more warily.