SK must read:*
Rose Madder - very good, scary accurate depiction of what DV does to the psyche
Duma Key - my favourite King, very scary and dark, SK's way of dealing with the trauma of his injuries, great insights into dementia, fabulous characterisations.
Under the Dome - actually, not like The Stand at all, a very good exposition of what is so scary about small town America
The Shining - Classic, must-read. The child's point of view is beautifully done (SK does seem to be good at children)
IT - Why has no-one mentioned IT? It's enormous, a big sprawling good-versus-evil novel, also a beautiful coming of age story.
Needful Things - More smalltown America, this time focusing on the downside of wish fulfilment. Recommended.
Salem's Lot - Vampires, done brilliantly.
And the Dark Tower series - takes a bit of getting into, but well worth the investment.
SK worth your time
Desperation (you will also need to read Richard Bachman's The Regulators to really get this - Bachman is King's alter ego)
Misery
Dolores Claiborne
Bag of Bones
The Green Mile
The Stand
The Dead Zone
Lisey's Story (disagree with a previous poster here, it isn't an easy one, but it is a beautifully done dissection of madness - what it is, and what it does to people)
SK novels to avoid (disagree with me if you wish): Carrie, the Tommyknockers, From a Buick 8, Pet Sematary, Dreamcatcher
Hope that helps.