This is a 26-year-old film that still freaks the bejesus out of you. Scary, suspenseful, gory. It still holds up and in fact probably trumps a lot of the same genre of the past 20 years.
It's good because it's simple. Other than the creature being an alien, there's nothing supernatural about it. It's only real defense is that it can shape shift providing constant cover. Otherwise, it's mano y alien.
By the way, you know in films with ghosts, aliens, supernatural shit, etc. how things start to happen and things are getting weird, but no one is just ever in shock that something so beyond belief is happening? Is that because everyone grows up watching films about aliens and ghosts? It's this fake reality that prepares us for the time when we're scientists in the South Pole and a man-eating alien starts killing us one by one?