This one isn't going to be much of a mystery when it comes to the ultimate grade, so I'll save you the suspense. He gets an A+.
I once read a quote by person attempting to put down Mr. King's work in which they said something along the lines of, "He's the McDonald's of horror."
I assume they meant that he's written quite a lot and that it seems made for the masses and thus isn't true literature. If you ask me, in the end, it's not much of an insult. If I could be the McDonald's of anything, I'd jump right on that opportunity.  Millions of dollars and millions of fans sounds pretty good to me.
Just because something is popular, doesn't make it bad. Just because something isn't Moby Dick, doesn't make it bad either. Stephen King books manage to itch some back, dark corner of your brain that needs a scratch every now and again. Sure there are probably lots of other creepier, scarier writers out there. I don't care.
I loves me some Stephen King books.
As the Pook can attest, I literally own them all.
And I've read them all.
He is one of my three favorite authors. 
While Mr. King is still putting out new material, it doesn't come out nearly as fast as I'd like, so to get an appropriate King fix, I've made the following decision.
I'm going to reread all of them.
Yes, all of them.
From the really good (Misery, Salem's Lot, Firestarter) to the really bad (Insomnia, Tommyknockers).
From the short (The Girl Who Loved Tom Gordon, Carrie, Thinner) to the seriously long (The [uncut] Stand, The Dark Tower Series).
From the tame (Eyes of the Dragon) to the downright creepy and dark stuff (All Dark No Stars).
And of course, I'll give it all a grade.
My current plan is to go chronologically. 
Feel free to read along if you'd like.
Stephen King: A+ 
 Heck, I'd be the Wendy's of anything.
 The other two are Dick/Felix Francis and Nick Hornby. I own everything by them as well. In fact if you are looking for something new to read, I would heartily recommend any of the three.
 I've actually already started Carrie. I'm about 40 pages in.
 Told you so. Also, I should probably be clear that I am just talking about the books. Some of the movies are good. I few are great. Most kind of blow. The TV shows don't fare much better.