John Carpenter's The Thing [http://www.amazon.co.uk/Thing-DVD-Kurt-Russell/dp/B00004D07X/ref=sr_1_1?s=dvd&ie=UTF8&qid=1302886934&sr=1-1], easily.
I think Alien and Jaws get the honourable mentions, and not necessarily in that order. Jaws would have been shite, but the rubber shark kept breaking down so all the cheesy underwater footage of a rubber shark they'd been planning to use had to be replaced with shark's-eye-view approaches and footage of open, seemingly empty, ocean, and that was scary.
Just my opinion, although I hope I'm far from alone in it: the films made of IT and Christine have both utterly sucked, and neither of them was even slightly scary or entertaining compared to the books. It might be possible to convey the idea that IT is chasing you and you don't know how clsoe to you IT is but are sure you don't have time to turn around and look, using cunning camera angles, tracking shots, zoom and so on. It might be possible to get the idea of IT having no shape at all as it came up the tube until Riched said it was the werewolf, without it looking cheesy. It might even be possible to come up with a genuinely terrifying Pennywise and some convincing deadlights. Likewise, given an adequate supply of the '58 Plymouth Fury and the autumn red and ivory white paints, it might be possible to do the self-repair thing, the scene where she squeezes her way up an alley and so on, and I'm sure someone could merge modern (well, late '80s) town into late '50s town, and it wouldn't be that hard to have an old-fashioned radio that played any of a number of (simulated) circa-1960 radio stations, possibly even from original recordings of real New England stations. Somehow, though, I doubt it would happen. The history of IT and the way it haunted people would be too much like Doing It Right for cinema, and the long, slow build-up as Roland D leBay's nature and Christine's nature became more apparent would take too long for the 112-minutes-or-under world of cinema. I think the films made of those stories sucked beside the films that could, theoretically, have been made the way Stalingrad sucked beside the miniseries Das Boot.