Stephen King is an amazing writer, and I've read nearly all of his masterpieces. Unfortunately, the movies based on his books don't even come close to showing his true artistic genius. I think the main reason is because he goes into such detail in his books (in order to give background history for his readers), and it's impossible to capture all of those important details on film. If you did, the films would be VERY long.
The best movie based on one of his stories is
Stand By Me. It was created from his short story "The Body" which is found in the book "Different Seasons." I highly recommend reading it.