Since we have veered into overall rankings of the Alien franchise, I thought I'd add my two cents because, as you might be able to tell from my prior posts on A:E, I have thought way too much about this franchise.
First- I don't consider any of the various spinoffs or Alien v. Predator movies or other stuff part of the franchise. At all.
Second, I think that there are two inarguable greatest movies in the franchise. Alien and Aliens. Period. Look, I get arguing for other movies and being all hipster, but those are the two classics. Of the two, Alien is the better one. That's not a slight on Aliens. Aliens is amazing. I love that movie. I'd argue that it's a more lovable movie than the original. But it's also an action movie- a great action movie, one of the best '80s action movies, but not a genre-defining horror classic that gave us a visual and film vocabulary that is still used today.
1a Alien
1b Aliens
Next ... okay, this will be unpopular. There's a lot I don't like about the next one. But I do like the ambition. I do like a lot of the specific scenes. I do like the cinematography. I do like the atmosphere. And to me, that more than makes up for the absolutely bizarre choices (too many to list, but ... don't run straight ahead, you can literally jump to the left or the right!). And ... some questionable lore. Questionable is a good word for it, right?
2. Prometheus
Following that is a film that is third in the trilogy, and third in the rankings (see what I did there). Look, I get the fan hate (killing Newt and Bishop after Aliens was harsh). I get that the studio kneecapped Fincher's vision and the budget to make this a truly great masterpiece. But ... if you can get past that, this is a good film. Imperfect, but good.
3. Alien3
Then there's what I call the J.J. Abrams Alien movie. That's right- you try to course correct the franchise by playing the classics. It's perfectly cromulent. Needz moar lenz flarez.
4. Alien Romulus.
Following that is the sequel to Prometheus. I don't want to totally bash it. Fassbender is a great actor, and he brings it. The opening is evocative and, um, kind of tricks you into thinking, "Hey, is this an Alien film?" It does try to wrestle with ideas and stuff. But ... it just does not work. It doesn't hang together, and it, um, doesn't hand apart either. That's just my opinion. But you see that I gave Prometheus the benefit of the doubt, right? I really tried with Covenant ... but nope.
5. Alien: Covenant
There is no other Alien movie. If you think that there was a movie after Alien3, then you're wrong. That was a terrible dream.