Speaking of masterpieces: Alien!
Alien is from a time when sci-fi films could be slow paced. Now everything has to move at a certain pace so morons don't fall asleep. Before pace could be an art form all its self, but now its like a formula and every film seems to follow it. The acting, design, concept, story, characters, actors, I just don't think this film can be improved on. Even the fact that none of the characters are over the top either. And Ripley might be the greatest female hero of all time, because she isn't over the top like modern females throwing guys over their shoulders and so on. Everything she does, how she reacts, her motivations, etc, are all believable. Great character and great actress. At least for the first 2 films.
And as much as I love the EXTENDED version of Aliens, I do think Alien is superior, and that Aliens makes an amazing sequel. Its a shame they did what they did with Alien 3, and don't get me started on the others. And def don't bring up Alien vs Predator. Garbage.