As far as the horror novels are concerned, there's no match for "Carrie", but I chose "Stand by me", because it's a) one of my personal dearest faves since the time it played theatrically and b) it really looks like the words in the novella transformed into images on screen. Plus, it makes some valuable points about childhood friendships and their impact in later life.
Generally I'd say that King's non horror fiction usually inspires much better movie results than the genre fare for reasons, which I'm not entirely sure about. ....