Aliens is my choice in the poll, but ranking it above Alien is tough since I think both films are really two halves of one story.
Anything after that is pretty much bonus content, though I also like all of the subsequent films - to varying degrees and for different reasons, of course: Alien 3 for its industrial bleakness, which just follows on perfectly from the first two films and for that ending; Resurrection for its total goofiness and weirdness; and the two prequel films for introducing everyone's favourite amoral android!
So to me, there's no bad Alien film. Just films that aren't as great as the first two.