The worst book he ever wrote (scariest) is The Shining. Scared the CRAP out of me both times I plucked up the courage to read it. And I read it in Jack Nicholsons' voice, which made it even worserer.
I don't know that I'll ever be brave enough to give it a third outing.
I've finished IT, and I'm half-way through From A Buick 8, and then The Talisman is looking good -- it's been a long time since I walked the Territories with Jack Sawyer.