Agree with all of the above, and it's the only film in the series to hit all those points. Aliens is a fine action movie, but that's a different beast altogether, and the alien itself loses its impact once mown down in the tens or the hundreds.
Anyway, the other thing I like about Alien is the realism. The environments strike me as much more what it would be like on a spaceship than most of the wonderful, spacious, squeaky clean ships we so often see. Similarly, the fact that it's boring work adds to that realism - no plying the stars with a new adventure every 5 minutes. Just tedious slog.
All that I think is what helps its immersion. This is a world you can relate to - if you work on a ship, an oil rig, in a submarine, down a mine, this is you. This is what your work is ALREADY like.
Same with the people, these are not gung-ho heroes, no typical movie stereotypes. These are "just people" and I think that also helps us relate, and so helps us really join them in their dreadful situation.
And of course the alien itself, a wonderful piece of horror design. At once beautiful and appalling, elegant and demonic, sexual and predatory, organic and mechanic. It remains one of the few things I have occasional nightmares about (well, they're not really nightmares in a sense as the dreams don't terrify me as such, but they are dramatic!) and I think the design does an excellent job of tapping into several parts of the human psyche. Kudos to Giger.
And then similar kudos to Ridley Scott for knowing how to use that design to maximum impact, marrying Giger's work to his use of light and shadow. Brilliant! (And when I watch Prometheus, I think "How far the mighty have fallen.")
The other thing was the movie's bold step in redefining a genre, in moving away from both the sci-fi and the horror of the day, and doing something very different. None of the regular hooks that audiences could relate to, from the movies and TV series of the day. That was ambitious, and the movie stood out then - and still does now, many decades later.
Anyway, enough rambling. Had to say something though, since you asked, and it is one of my favorite movies of all time