My vote is for 'It', I've read it every year since I was fourteen. There are so many layers to this story, and the perspective of all of the 'losers' as children and adults is great. I particularly love the inserts from Mike's diary, where he discusses the history of Derry. Some of the stories he tells are the creepiest I've heard.

I agree that 'The Long Walk' is one of his best short stories. I find his 'Night Shift' collection to be the scariest of the short story sets i.e. Graveyard Shift, The Boogeyman, Sometimes They Come Back, Children of the Corn etc.

My favourite of his short stories was 'The Ledge'. Not a horror really, but it's interesting to be inside this mans head for the few minutes he is outside walking the ledge.