I have to go with Salem's Lot, just because it's scary for two reasons: one, the vampires are well done, and two, the portrait of a small town eating itself alive is really excellent. The book of King's that I think is most likely to survive a hundred years. Also I like The Shining, The Stand, and Misery. And Koontz is a waste of your time no matter what you read, except Watchers and Strangers.
In my opinion all his best books are from the 70's and 80's. He started to suck in the 90's he didn't have one true classic from then on till today.
"It" scared me away from sewer grates for several years, but in terms of overall quality, I'd have to go with "The Stand". I actually like some of his recent stuff, especially "Bag of Bones" and "The Green Mile". They're more literary and less scary, but still quite good. I think they'll be teaching "Green Mile" to high school kids a few decades from now.