Be warned of unmarked spoilers.
Some might say that the film Alien is overhyped. That it isn't really so terrifying or brilliant as some might say. There might be some truth to that, but simply because people give it great hype does not take away from the fact that both as a story and a story to scare, it does very well.
As a story, it manages to generally follow two rules. The first is that the people in it are humans, most of them anyway, and the second is that as humans they act as rationally as they can for their perceived self interest. They do not wish to enter dangerous situations, they are compelled to by circumstances outside of their control. They do not simply blindly rush to fight the Alien, but rather formulate plans and act as duty says they should. When they do not act as rationally as they should, there are understandable reasons for why they do not. At different points the captain wishes to save a member of his crew from horrible death or a crew member is driven to helplessness by simple human fear. When they conflict with each other, it's for their personal human self interest. It is that reason and humanity that makes it actually interesting to follow them, and also why it makes the Alien such a good monster.
Much like other memorable villains, the Alien is a threat in spite of the logical actions of humans. It may have some passive help from an android's orders to retrieve it, but honestly watching it in action the viewer is left with no doubt that it could do just as much damage on its own. In that is the terror. Humans, trying their best and coming up with different strategies for what to do, just aren't good enough. Add to that the memorable horrifying image of sexual assault made flesh courtesy of H.R. Giger and you have a threat to attack the viewer on a very intimate level.
For most this is probably not the ultimate movie experience, but it definitely should be regarded as a very strong work that sadly was followed by movies that for the most part were of lesser quality at best.