Alien (1979)

Alien is basically a haunted house film set in space, and it's still one of the best. The crew feel like real people doing a boring job (they're essentially space truckers arguing about bonuses), which makes what happens to them all the more terrifying. The spaceship feels lived-in and believable, and while the alien itself is obviously speculative, its life cycle isn't that far from some real parasites on Earth - which is somehow even worse. Interestingly, the film was rated X in the UK on its original release - not because of aliens jumping out of anyone's chest, but because censors were concerned that the reproductive cycle of the xenomorphs might confuse teenage minds. I reckon that anyone learning about the birds and the bees by watching Alien has got bigger issues to worry about.