Not a short story, but shorter than a full novel: The Long Walk. It's one of his absolute best, in my not at all humble opinion. There has only ever been one story, of any length, that I didn't like and that is Gerald's Game. That one is way too real and could happen, probably has happened.
I haven't read enough of his short story collections to rank but I'm currently reading Skeleton Crew and am liking a lot of them so far! They're more fantasy than I expected with the right amount of grossness.
The one with the dad that turned into blob, the one with dying grandma and the one with the guy who kidnapped the kid who waited for his popsy or something.