Even though I feel the original Alien is superior to all, my favorite to watch is Aliens. It has it all; action, suspense, plot, etc.
I used to dislike Alien 3, but after watching it when it was first released on VHS I grew to like it. However after growing up and learning more and more about it, I've once again fallen into the dislike (but not hate) the movie. The biggest thing for me, is the situation with the alien seems to play second fiddle, which wasn't the case in the previous two movies.