I would have to say as a person who has read a good majority of his books I think Misery is the best adaption without changing too much from the story.
The Shining was good but the book and the movie weren't a lot alike at all. The book was far more psycological where as the movie just devolved into a sort of semi slasher flick with an axe.
It is just a terrible adaption. Simply awful. It's very little like the book (though the book is incredibly long and has some sicks parts in it regarding the children and if you've read it you know what I am referring to.....). It's funny because I found the movie scary and then I read the book and saw the movie again and it looked like straight up clownery to me (pun intended).
Carrie is a pretty good adaptation as well even though it has a similar problem like the shining where it just became a horror movie instead of examining some of the ideas King did in the book.
Stand By Me was a good adaptation as well IMO. I think that is actually King's favorite of the lot of his own books to movies.