Aliens: Ripley vs. the Mother Alien
How do you improve upon a Ridley Scott classic that changed the face of sci-fi? Have James Cameron write and direct the sequel. Oh, and make the "Alien" in question plural. Nicely done. Along with a certain other part II (er, part V) later on this list, this epic 1986 sequel could qualify as the Godfather II of sci-fi action films in that many feel it's better than the original. Whatever your stance on Aliens, when Ellen Ripley (Sigourney Weaver), defending her little pal Newt, screams "get away from her, you bitch!" at the mother of all aliens (literally), you know it is on.