I like all the orignal four
Alien movies for different reasons. I also like the fact that each film moved the story onward, and didn't just repeat the formula of the first.
Alien obviously created the horror that infests
Aliens, the sequel which really opened up the possibilities.
The weakest, for me, was
Alien3 since it was so visually depressing. After the excitement of the previous film it felt like a less effective version of
Alien. There was very little uplifting in this one, though it did move the story in an unexpected direction.
Alien Resurrection gets a rough ride. Though this one is pure Dark Horse comic book. It's the
Star Wars Original Trilogy of the Alien series. A space opera on a larger scale than the former three, complete with black comedy, visceral horror, swimming aliens, Ron Perlman and Winona Ryder. Who could ask for more?
The first four gave us four different views of the Xenomorph (and human) menace. If you buy into the story then they work, but if you look for four versions of
Alien then you're going to be disappointed. Four versions of the original would have been as cliched as a series of slasher movies.
If you include the AvPs and the Predator movies, this becomes a massive inter-related story - and that's how I like to see them.