AVP and AVPR don't count. They suck anyways.
6: Alien Resurrection
Was a parody of the alien saga, had one dimensional characters, fun dialogue and serious story don't mix, completely unnecessary story, and you don't need to even watch this movie in the first place.
5: Alien 3
I actually liked Alien 3 and thought it was a good conclusion to the the Alien trilogy (shouldn't have been a quadrilogy). However, it's depressing tone, indistinguishable characters, bad blue screen effects, and the choice to kill off Hicks and Newt (spoilers) at the very beginning just worsened the movie.
4: Prometheus
Unlike a lot of people, I loved Prometheus and I thought it was a risky move for the franchise that payed off pretty well. That doesn't mean it's a perfect movie as there are numerous plot holes, with only some being answered in its sequel, stupid decisions being made by professionals, and alien creatures that I thought were cool, but aren't even related to the normal xenomorph, as shown in Alien Covenant. It is one of most stunning visual movies of the franchise and the soundtrack is excellent.
3: Alien
The original classic of the franchise, Alien works perfectly as a suspense horror movie and the characters are great, however, the soundtrack being almost gone from the movie, along with the scare factor being reduced substantially on repeat viewings lowers it down.
2: Alien Covenant
My expectations were very low for this movie as a lot of people said it was just mediocre, but I was pleasantly surprised by this film. Alien Covenant is breathtaking with its locations and visual effects, on par with Prometheus, and the soundtrack is a great homage to Alien and Prometheus. I really liked the way of how the xenomorphs were created and the David and Walter conversations were great. The neomorphs were also an amazing addition to the species and is probably second only to the classic xenomorph. The characters are still stupid, but are also more likable, being married couples.
1: Aliens
Aliens is the perfect sequel to Alien going from suspense to action in the horror genre. Although the xenomorphs lose some of their terror, being killed so quickly, their huge hive mind and the addition of the Alien Queen makes up for that. The characters are probably the best they've ever been and the story is great.