Regarding Alien movies, Alien is the best, hands down. Aliens is a close second.
Prometheus is next in my ranking. It has many of the best qualities of Ridley Scott films with its production design, cinematography and lighting, etc. It does have two glaring problems - 1, stupid characters doing really stupid things, and 2, no alien. On that second point, the film was meant as the beginning of a series of movies that would eventually get the audience to Alien, yet it was marketed more as an Alien movie (even used that same screeching sound that scared the **** out of me every time the TV would play that 15-second ad on late night TV during the summer of 79). The marketing was not on Scott; more than likely, that was a 20th Century Fox decision. On its own, the movie is a damn good science fiction flick. David the android is one of the most evil characters ever put to film.
Alien Romulus is next. I particularly enjoyed the look at life on a WY outpost, which had only been done briefly in the director's cut of Aliens. I also loved that they confirmed something I'd long suspected, that the alien that Ripley ejected from the lifeboat could have survived the vacuum of space. The hybrid at the end was icky, and I didn't really like it on first viewing, but it was far better than the abomination at the end of Alien Resurrection.
Alien Covenant is next. I didn't really like much of the actual alien action in this one, particularly the chest burster that broke out of the Captain and was already a miniature fully-grown alien rather than the snake-like thing we've seen in all other movies. But I loved the opening between David and Weyland, I loved the anti-David in Walter and then David was still an evil sum*****.
Alien Versus Predator is my next choice. Yes, it's silly. Yes, it's inspired by a comic book (Dark Horse comics that were published in the late 80s). But like the original King Kong vs Godzilla, it's just one of those cheesy great fight movies where you just go watch these critters slug it out.
I find no redeeming qualities to either Alien Resurrection or Alien vs Predator Requium.