It had everything a great horror movie should. Likable characters, who slowly get destroyed by an evil creature. Creepy atmosphere, that alien ship was straight out of HR Giger. An alien bursting out of a guys chest. A creepy robot who might just kill everybody. An evil corporate lackey who might just get everybody killed. One of the most terrifying screams (by Harry Dean Stanton). A bad ass lead woman; Ripley (Sigourney Weaver). And, a pissed off cat.
So Alien, of of the best horror movies of the 70's & all time.