Dude, don't ask me to choose between them. That's just cruel. They are both absolute classics, and so different in tone, with Alien focused more on horror and Aliens being more action-oriented.
If I was forced to choose, I would give a very, very slight edge to the original for what it did for cinema. If Star Wars helped legitimized sci-fi in general as a serious genre, Alien did so for sci-fi/horror in specific, taking it out of the realm of B-grade (or worse) cinema. It made the sci-fi monster movie a respectable sub-genre. It also gave us perhaps one of the the first, if not the first, notable example of a female protagonist in the genre that was strong and proactive, and not just someone who screamed in abject terror at the monster waiting to be rescued by a man. Aliens, despite being an equally good film in terms of quality, wasn't as revolutionary of a film as Alien.