Ok, here is my list.
The first place goes to alien and aliens.
I cant choose between one of them and they are just to hard to compare, i cant compare a action movie to a horror movie. However, both are masterpieces without flaws.
The next one is an easy pick: Alien Ressurection.
Why? Well, at first i like how the aliens in this movie are fully developed personalities, i truly believe that THIS is the movie which formed the aliens to the thing was they are today. I believe that the aliens are the main characters in this movie. And Ripley, of course. I like the emotinal tention between the aliens and ripley and how it gets somehow emotinal when the newborn dies. However, it has many flaws: At first, its disgusting as hell, most of the script is lame, the human characters are not interesting and the clone idea is somehow weird. Also, it doesnt help that the aliens look like mutated dinosaurs. But, i think its a 6/10 movie.
At the last place comes Alien 3. I hate this movie. I dont like anything about it. The special effects are weak as hell, the script is a pure joke, it completely destroys the point of "aliens" and my toilet has better developed characters. The alien itself is a joke and the violence doesnt save anything, it just makes it worse.