It's listed as a sci-fi horror movie in many sites.
In light of that, what made the first movie remarkable is the use of suspense, with the creature not fully revealed until the end.
Since the second movie couldn't repeat that, it focused on action. Similar took place for the third and fourth movies.
With the prequels, the focus was on the origins story, with elements borrowed from the first few movies, like another landing and exploration and a heroine who looks like Ripley.