There is only one Alien movie that you need to own and it is directed by James Cameron.
You can't be this stupid
Alien is a classic film. It's a sci fi gothic horror that solves the ultimate problem with the haunted house, ie "why not just run out the house." In Alien there's nowhere to run. Alien is the second scariest film I've ever seen.
Aliens is the blueprint for a badass sci fi action flick, and while it's easily more entertaining than the first film it's certainly not superior. TVC obviously disagrees, but both films are essentially 10/10s for me