Alien deserves it simply for being so wholly unique. There was NOTHING like what Scott created before that movie... It's often pointed out as following the slasher outline, but seeing as Halloween (the progenitor of the entire slasher formula) came out not even a year before Alien did, it managed to come to the same exquisite formula almost in a vacuum. This is the film that defined the longstanding connection between Sci-Fi and Horror for the final decades of the 20th Century.
And even moreso, it's just an incredible film. The character work is fully formed, and Ripley is by far one of the best heroes of the genre (I'll admit that most of her badassness comes from her further development in Aliens, but still).
The original all the way.
