It goes without saying that the scariest movie ever made is The Exorcist. Audiences threw up in movie theaters across the country when it opened and some people even had seizures after seeing it.
My parents let me rent it from Blockbuster when I was in 4th grade and it gave me nightmares nonstop years after that. I remember seeing the re-release in the theater one night after a BC football game senior year, where I had to sit next to Arvind and Megan Neher, as if that situation wasn't already scary enough.
But while The Exorcist easily wins the scariest award, the film itself isn't necessarily all that clever or ground-breaking for horror films. Sure, it used some cool special effects and pushed verbal (and physical) vulgarity limits to unprecedented heights, but it was ultimately a strict book-to-film adaptation that remains in a narrow genre of its own that can't really be expanded upon.
So as far as the smartest and most influential horror movie of all time, that award goes to John Carpenter's Halloween. Produced on a shoestring budget, it was a massive sleeper hit released in the fall of 1978. It built and expanded upon the slasher film model introduced 18 years prior by Psycho and created an entirely new genre of horror movies.
It's true that just about everything that came after Halloween sucked really bad: Friday the 13th, Nightmare on Elm Street, etc. were all terrible because they lacked the darkness and depth of their archetype. Still, Halloween deserves credit for starting that genre in a manner that combined simplicity (good vs. evil, the boogeyman is scary) with creativity (evil in human form that cannot be killed, that wears a signature mask over a face we never get to see, that moves subtly and slowly but is unstoppable, that returns to haunt a particular town on a specified date each year, and that instantly appears/disappears in dark corners created by perfectly positioned camera angles or is often viewed from a first-person perspective).
Halloween is also great because it employs a less-is-more approach. There are very few special effects used and only a couple of bloody scenes. Except for the short prologue, we don't see the first killing until an hour or so in. In this way Carpenter, like Hitchcock, is able to slowly stoke the audience's suspense level instead of desensitizing it with the up-front, nonstop gore and guts scenes we see in theaters today. Carpenter also got lucky in discovering then-unknown Jamie Lee Curtis and casting her as the protagonist. There's just something about Curtis' performance, particularly in the last 20 minutes, that announces to viewers that something is going on here that clearly separates this film from the typical, campy B-movie category under which many of Carpenter's other endeavors fall.
So this weekend, forget paying $8 to go see Saw or whatever it's called. Head down to Movie Gallery and rent this timeless classic instead.