Poll
26%
17%
4%
0%
13%
9%
0%
9%
9%
13%
Well "Greatest" is going to be based on opinions. What shouldv'e been asked is "What is the single most important horror movie?"
Jaws in 1975 was the first of it's kind. IMO, Jaws sparked more interest in terror and suspense than The Exorcist and Night of the Living Dead/Dawn of the Dead (all of which were pretty much the scariest films around at the time). The next film to premiere whose fear was in the caliber of Jaws was Alien in 1979.
Jaws and Alien were rated by fan voting as the number 1&2 scariest movie of all time. As a few years passed, Carpenter made The Thing; one of the most frightening of all time.
I think the "Most important" (by which I mean the most influential) comes down to Jaws, Alien, and The Thing.