Alien first, then Aliens and then the rest.
Alien Resurrection is a special case. I like it, maybe because I'm French, and Jean-Pierre Jeunet realized it. More seriously, I like it, but I think of it more as a spin-off than as a sequel, for many reasons of style, tone and story. Alien 3 is to me a valid end to the series, that would thus be a trilogy. The assembly cut makes the movie better, but alas it spoiled its potential, in my humble opinion. As many other fans, I think that Hicks and Newt's deaths were too much. Aliens is a beloved classic now. But Alien... Only the designs by Giger and others, the Derelict, the Space Jockey, the Alien, the Nostromo, are masterpieces. The movie is as cleverly built as its monster is scary and at the same time marvelous and intriguing.
I hope that Prometheus will be at least equal to Aliens in my scale, but if it manages to get on the same level than Alien, I will be pleased...