The four movies together made an interesting ensemble piece. I appreciated the Ripley character. Alien was my choice for the best because it piqued my curiosity of the aliens' origins, and that alien ship was so damn cool.
I didn't enjoy the second one. It was okay at first, but once they introduced the kid it was all over. I read the kid's presence as an attempt to increase my sympathy for the good guys, and it didn't work. On top of it, the kid's presence turne Ripley, an otherwise decent action/horror/thriller character who just happened to be female into a mom-character. Can't have a female without grasping for one of the virgin/whore/mother angles, eh?
The third one was a relief, mainly because that damn kid wasn't in it. Otherwise, it left little impression on me.
I enjoyed the fourth movie. While I do not have a natural affinity for life sciences, I respect them immensely. The genetic engineering stuff was both visually and mentally stimulating. I especially liked this movie because for me, it redeemed the mother-role in the second movie, sort of turning it on its head.
I would really need to rewatch these if I want thoughtful, developed opinions.
What did everyone else particularly like/dislike?