Stephen King's legacy will be vast, I have no doubt. We'll still read him hundreds of years from now, just as we have with Poe and Dickens and many others. Of all his master works, however, I take the somewhat unpopular stance that Pet Sematary is his magnum opus. Re-reading it now only confirms this opinion.
When I first read Pet Sematary (I couldn't have been older than 13) I knew right away that it was more than a typical scary story. For one, it made me feel decades older. Wiser. More attuned to human nature. King never shies away from character, but he really digs deep with Louis Creed. There are numerous novels that portray death well (James Agee's A Death in the Family is superb) but fittingly enough, it's this Gothic horror novel that illustrates it best. Death isn't pretty and surviving it can be just as grotesque. Pet Sematary gives all of this to us, and more. Much more than we want to see. But maybe we need to see it to understand.
We often scream at characters in horror movies for doing stupid things (WHY WOULD YOU LEAVE THE HOUSE YOU IDIOT!?) and arguably Louis Creed does some stupid things in this book. King adds supernatural influence as justification, but let's be honest - no justification is needed. Creed and his decisions are as relatable as they are tragic, which is something never quite accomplished--not on the same level at least--with Jack Torrance or Annie Wilkes or Carrie White. Not dissing those other books, I'm a fan boy for them too, but it's why I think Pet Sematary is King's greatest achievement.