Why is Alien the greatest space horror film of all time?

For one thing, it does a fantastic job of making the crew of the Nostromo relatable and human. We spend a lot of time with them before any seriously suspenseful or horrific things happen. We understand they're a tight crew, and care about each other, and that they already feel some animosity towards the mining company that sent them out into space. This makes it a lot more scary and sad when eventually most of them become prey to the alien.

Secondly, the main villain in Alien is not only way scarier than almost any other creature villain ever made, the way they build up both her backstory and her eventual full reveal is amazing. When we first see the small chestburster alien, neither we nor the crew have any idea how big, terrifying, and lethal she will become. From her sticky, slick sheen to her cunning and silent hunting skills, she is truly an unstoppable force. If I were trapped on a spaceship with that creature, I don't know what I'd do.

And finally, Alien just feels more "real" with its universal and timely themes of greed and fear of the unknown. There are numerous actual stories of mining companies betraying their field employees and endangering their lives for a quick buck; isolated teams of workers going crazy,  suffering from unseen maladies, and falling into chaos and disarray; and of course, the tale of predator and prey is perhaps the oldest one of all. Its visceral, brutal, claustrophobic terror felt imminently possibly in the 1970s when it was made, and has only increased with time.