Alien.
If it wasn't for that silly, cliched (4th wall breaking as a result) shower scene, and a over-compensating to the Aliens fanbase final act, I might say Covenant. If it wasn't for the dumbness of some of the characters, Prometheus might be up there. If it wasn't for Alien3 being confusing as to who was dying, and comedic in the wrong places, I might say Alien3. If it wasn't for Aliens making Ripley implausibly unscared of getting near and taunting the aliens, I might say Aliens. If it wasn't for the fact that Resurrection, and Ripley's clone's instincts, are too grotesque, I might say Alien Resurrection.
Some other movies are scarier for longer, or do more in visually exciting ways, but Alien arguably does least wrong. But I admire all of those movies for the very different things they convey.