I'd say I'm a fan of Stephen King. I've read 23 of his books, including On Writing and Danse Macabre, his only non-fiction as far as I know. Of the books I've read, I think I've enjoyed almost all of them, although King has a tendency (especially nowdays) to stretch a 30 page story into a 300 page one. I said that to say this, though: I've never been scared by one of King's books or short stories until I read Pet Sematary.
The plot is very simple, which is probably why it's so effective: Louis Creed to the town of Ludlow with his wife Rachael, his son Gage, and their pet cat, Winston Churchill. Soon after arriving, Winston Churchill is struck by a truck and killed, and Louis' old neighbor, Jud Crandall, tells Louis a secret. Behind their house, there's a cemetery. Animals buried in it come back to life. Louis buries Winston who does indeed come back, but something isn't quite right. One day while playing, Gage is hit by a truck and killed. You can connect the dots here.
The real horror of Pet Sematary doesn't really come from the plot, which is as old as the art of scary stories themselves. Be careful what you wish for. Don't tamper with cemeteries. Sometimes dead is better. It's the atmosphere, which is considerable and completely unrelenting, and in the realistic depiction of the Creed family as they struggle to deal with the death of their son. The book's climax, the resurrection of Gage, is both its centerpiece and, somehow, beside the point. Next to the horror of seeing your baby son run down, what's a zombie or two?
Pet Sematary is completely unmerciful. It never cuts its characters or its readers a break. Something about it is fundamentally disturbing in a way most of King's work (and most horror in general) isn't. It's nihilistic and hopeless, and that's why it works.