ALIEN. Hands down. Defined the genre, and as Dan Obannon says, closed the book on it. It had everything it needed. Aliens is fantastic, among the best sequels in movie history, took what Alien did and made it better, took it further, expanded on Ripley and gave us awesome new beloved characters. But Alien started everything and was a true innovation. Predator is awesome, but without going into loads of detail, sci-fi horror kind of trumps sci-fi action for me. Beyond that, none of the films really contend for 1st place (though I love them all for their own reasons...except AVPR and Ressuraction 😑😑)