Space. The endless, expanding universe that hides all manner of new, unknowable creatures. Way back in 1979, Ridley Scott showed that we should be careful poking around up there, as we might not like what creatures we could encounter.
For me, Alien is a masterclass in suspense. The atmosphere aboard the Nostromo is one initially of camaraderie which slowly disappears as the crew grow more and more frustrated with one another. Bringing an impregnated Kane aboard invites the xenomorph inside the ship, but this is just the flashpoint for an already tense environment just begging for some sort of flashpoint event.
The main reason Alien works so well for me is the atmosphere. The Nostromo spaceship is a bleak, uncanny environment. Very early on the place is damaged by a rough landing, plunging a lot of the ship into relative darkness. Those roomy, white-washed corridors suddenly become the perfect haunt for a pitch-black, silent, stealthy killing machine. Derek Vanlint's camera creeps through and around corridors and vents in slow, predatory movements, and switches to jarring shaky-cam when the alien is cutting its way through the crew at the end of the film. Consequently, we become fully immersed in the Nostromo, making us almost at much at risk of alien attack as the members of Dallas' crew.
In an age where continuous alien sequels are relying more and more on increasingly gory deaths, the xenomorph's kills in Alien might feel rather muted. But this is purposeful. We see very little of the creature so that we do not know much about it whatsoever. In this first film, we as an audience have no concept of what this thing can really do; all we know is it must be relatively intelligent in the way it stalks its prey around the Nostromo so effectively. On my first watch, I found myself constantly looking for the titular alien, constantly mistaking hanging chains or unfamiliar spaceship architecture for the biomechanical design of space's apex predator. As the crew slowly uncover the alien's physiology and hunting methods, it only becomes more adept at evading and attacking the crew.
Obviously, it would be silly not to mention the practical effects on display, and they still haunt me to this day. The chestburster scene has endured as an iconic moment in cinema, and each film has tried in one way or another to emulate it. The way Kane's stomach bulges and writhes before the juvenile xenomorph bursts out of his torso shocked cinemagoers back then, and definitely shook me the first time I saw it too. HR Giger's horrendous design work is so unique and distressing and stands head-and-shoulders above the numerous redesigns and subsequent iterations of the alien subspecies that have entered the creature's canon since. No effects, digital or otherwise, have ever rendered an Alien creature as this first foe.
Finally, you cannot watch Alien without marvelling at Ripley. Sigourney Weaver's legendary protagonist is beloved for a reason. Weaver is entirely put through the wringer as she is pursued by (and eventually beats) the original alien, a moment which becomes only the first of her many exploits involving the predatory alien creatures. Ripley shattered cinematic norms when she first appeared onscreen and has endured as a landmark role not only for Weaver but for female protagonists generally.
Alien will continue to endure as a landmark sci-fi film, for good reason. The grim, overtly sexually violent way in which the xenomorph race preys upon humankind was and will remain, truly shocking.