Alien is quite entrenched in the vernacular of science fiction fans, and of movie fans in general. If by chance you have not seen it, make sure to do so. It is about as good as it gets.
If you’ve seen any of the sequels (Aliens (1986), Alien 3 (1992), Alien Resurrection (1997)), try to separate in your mind this original Alien from the followers, and consider it on its own. You should also bear in mind when it was made — 1979 — to understand what a seminal film it was.
The original Alien is an essential film of the sci-fi genre, and universally recognized as such. Therefore, there is already an endless stream of praise and criticism that’s been written about it. At this point, I will add no more. A terrifically great film.