What makes Alien great is it's synchronization of sound, visuals and showing great attention to detail. Not only that but Alien has an unnerving atmosphere right from the beginning. We're introduced to a crew who is not exactly too friendly to each other add to that the ship. It's huge, and with its halls and corridors, it feels almost like a maze.
Enter Alien, a ruthless parasite with humanoid form yet it lacks any civilised traits of a human. It does have humanoid form but it doesn't give a single emotion. The design of the monster is what's the most terrifying (and it still holds up despite Alien being released in 1979). The creature is also better adapted to the ship's interior making a big part of the movie feel like a sinister game of cat and mouse.
What I particularly like is how the characters are written. They are not Hollywoodized heroes, in fact, there is no hero. They're just people which makes them even more threatened. The performances are all equally realistic and do deliver. Kudos to director Ridley Scott for using crafty cinematography and combining it with shadows and lights in an eerie way. The music is unrelenting and combined with Scott's cinematography, sound and visuals it adds to the eeriness.
The plot is also not another Hollywoodized cliché. It's more like "survival of the fittest". The characters battle true terror as they race to survive or outlive each other, they're all just a part of a race to see who will prevail and who will die.
Final Rating: 10/10