the reason why the stalker is scary is because usually enemies in survival horror games arent that smart. yet here is an enemy that was intentionally designed to be smart and make use of the environment to kill you. Basically its  ai is what makes it most frightening.

stalkers are very clever, they never show themselves until they are ready to attack while the others run around shrieking to throw you off guard as they also hide behind objects so you don't have a clear shot, stalkers are a pain in the ass, so far they are the scariest enemy in dead space(or 2nd compared to that thing that always would regenerate in the the first game)

Exactly!  Couldn't have said it any better myself, the AI of the Stalker in Dead Space is honestly some of the best enemy AI i've ever seen in a game, very very impressive.......and terrifying.

Bubble Bobble final boss is scarier, I dont even now wtf is that, and you can see al the skulls around you, even the platforms, is like they are asking you the climb up to your death.