In my opinion, his best book isn't horror. Check out Eyes of the Dragon, a fantasy novel he wrote in the late 80's. It does contain his favorite antagonist, Randall Flagg. I also really enjoyed what I've read of the Dark Tower series, but I haven't completed it.
If you insist on a horror novel, I recommend Needful Things. It's a little faster paced than most of his novels. I might not be the best person in the world to ask though, because I really don't care that much for Kings writing style. Pet Semetary is also good, but it's so slow it takes an iron constitution to make it to the payoff.
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