Alien

Start here, or start with Prometheus if you want the chronology, but understand what you give up by not starting here. Every other film in this series is a reply to this one. Aliens argues with its restraint, the prequels argue with its mystery, and Romulus is an attempt to rebuild its exact texture forty-five years later. Watched after any of them it reads as the quiet one. Watched first, it is the film the others cannot get out from under.

What makes it work is how little of it is horror. The Nostromo crew are contractors hauling ore, and the first act is a pay dispute. Brett and Parker want their shares and keep saying so. Nobody is an explorer, nobody is brave, and the ship is damp and badly lit the way a working vessel is rather than the way a haunted house is. By the time anything happens you have spent forty minutes with people complaining about their jobs, and the creature arrives into a world established as ordinary.

The other half of it is a refusal to explain. The derelict ship and its pilot are never accounted for. The creature has no origin, no motive beyond itself, and no biology the film is willing to describe. That withholding is the asset the sequels spent decades drawing down, and Covenant eventually spends the last of it by naming who made the thing.

One practical note. The 2003 release is a Director's Cut in name, but Scott trimmed as much as he restored and has said plainly that he considers the 1979 theatrical version the better film. The cocoon scene lives in that cut and is worth seeing once. Watch the theatrical first.