The Thing
This is one of the most compelling horror movies there is. The Thing sets the bar for cosmic, alien horror, and it's qualities, despite being a movie made in the 80s, still hold up perfectly fine today.
The idea of the movie already keeps you in suspension, with an alien that can disguise itself as anyone, leading you to not know who to suspect, especially with how the movie gives you little to no hints on who's infected or not.
The setting of the movie plays an especially important part, with it taking place in a remote station in Antarctica giving the movie more of a dreadful atmosphere knowing that there is an alien among these select few people, and them having no contact with the outside world, leading to them having no choice but to confront the alien by themselves with whatever they already have at their disposal.
The Thing in the movie is the pinnacle of practical effects, with it's grotesque, horrid body composed of dead animals and humans making you feel the fear of how alien and terrifying this Thing really is.
This movie, this Thing, sets the standard for how terrifying aliens can be in film, and the horrors that can be found in the cosmos.