Did a rewatch of the Quadrilogy, and Alien remains my favorite. I mentioned elsewhere that Alien has a spiritual aspect of horror that the sequels IMHO do not. The Alien feels more like a destroyer demi-god, designed to wipe out mankind in the most terrifying ways; turning all aspects of eros into death: kiss of death, intercourse of the mouth leading to a birth that leads to death. The Sequels tend to opt for horror and gore, and or action; but Alien feels like it penetrates the soul; the dark temple of eggs in The Derelict feels familiar and foreboding (as if we've been there or are going to be, or something in between), the Xenomorph beckons and disgusts; like man it is beautiful and violent; you in one way feel like Golic in Alien3 saying "magnificent!," and on the other hand like Ripley in Aliens, "I say we nuke the site from orbit, its the only way to be sure."
Got carried away...