Normally I think the original movies are better than anything, but I like Aliens (#2) is best because Ellen Ripley knows her stuff. She knows about the aliens while in the original, she was oblivious. And I like Vasquez. This bad a** female in aliens. There's another female in that movie, but she isn't in it as much I don't think. Or at least isn't as popular.
I have all 4 (Quadrilogy) in a box. Alien, Aliens, Alien 3, Alien Resurrection