Alien / Aliens. Interchangable for me. There are both such different styles of cinema, yet both concepts are both so strong. Aliens is very different from the claustrophobic horror of the original, and instead replaces it with spacious terror through sheer numbers of the creatures. Aliens built so strongly off the first film, does its own thing while honoring the concepts of the first movie and even the beats and guidlines of the first film. Alien is just pure sheer horror. It's the epitome of sci fi horror. But Aliens is this dark, twisted action epic. I think the sequel is more fun, but the first is very frightening. I voted for Aliens sheerly because at the end of the day I can watch it anytime and be entertained, as opposed to having to be in the mood to watch Alien.