Alien is a Ridley Scott masterpiece. It shook its audience when it came out several decades ago and it still does the same now. Any aspiring suspense director would do well to study this movie in depth.
The good. What makes this film as close to perfection as possible is the attention to the setup. It starts during the initial credits during which the simple visuals as well as the score puts you in the mood. It's slow and heavy with chills already. Then you get to know the environment. The crew wakes up, and you get to know them...
The pace throughout this flick is perfect. It's a slow climb to an ultimate peak of tension. The dialogs, the characters, the camera work, the editing, the script, the story, the background history... everything is meticulously done. Most special effects still hold the road even now, because they were extremely well done and used correctly.
The actors. Great selection for the cast, each member well adapted to its role. I'll simply mention the excellent work of Sigourney Weaver who's career was launched with this title, and Ian Holm who's performance is truly unforgettable.
The bad. The only reason I didn't give this movie a perfect score was because some of the technology is getting old. The size of the main computer. The many blinking lights. And the text interface, to some extent. Only two special effect sequences are really getting old. The planet-side landing of the ship. The triple explosion with the sheet of fire.
The rest can easily be imagined as some sub-standard rough industrial technology, a bit like what was done in Serenity (review yet to come), the crew having to deal with less than ideal conditions or hardware.
The ugly. Nothing. This is very close to perfection, if somewhat old.
The result. Horror, shivers, and sweats guaranteed. See it.