Aliens- Fast paced, suspenseful at points, exposes you to a lot more technology and the Queen (and the bad ass marines), possibly some of the greatest dialogue in any movie.
Alien- Suspenseful, dark, creepy, a little bit of a thriller, exposes you to a whole new universe and genre of film. (The only reason it's behind ALIENS is because I'm more of the action/suspense/thrill type of person, otherwise they're both 10/10 for me)
(Prometheus at number 3, if you count it as an Alien film)
Alien 3- Very cool alien breed, but other than that it really doesnt have much else going for it, it killed off all the people from Aliens and really just was a shitty mess for what it could have been.
Alien Resurrection- Completely ruined the Aliens franchise, made it seem like a joke, and because of that I honestly think we will never see another perfect Alien film like the first two.