Aliens

Unskippable, and not for plot reasons. You could follow the series without it. You cannot understand the series without it, because this is the film that established that an Alien film can be any genre it likes, and every entry since has had to pick one.

Cameron's move is the one nobody expects to work: he does not escalate the horror, he changes the category. The first film is a haunted house. This is a combat picture, with hardware, a squad, an insertion, an ambush and a defensive siege. The creature stops being an unknowable singular thing and becomes an enemy force with numbers and a nest. That ought to destroy what made the original frightening, and instead it produces the most quoted film in the series, because Cameron gives Ripley the one thing the first film denied her: knowledge. She is the only person in the room who knows what is coming, and nobody listens until it is too late.

Watch the Special Edition if you have the time, and know what it changes. It restores the scene where Ripley is told her daughter died of old age during the fifty-seven years she was adrift. Without it her bond with Newt plays as instinct. With it the bond is a replacement, and her refusal to leave the girl behind becomes the second time she makes that decision rather than the first. It also adds the colony's sentry guns, which are the best suspense sequence Cameron ever shot and which the theatrical cut leaves out entirely.

The ending matters because the next film undoes it. Ripley, Newt, Hicks and Bishop get out, and the film closes on something close to peace. Hold that in mind for about four minutes of Alien 3.