The Stand. Its less horrory than most of his books Id say, though its obviously disturbing, so if you like that kind of supernatural horror stuff maybe its not the way to go. And its incredibly long, which is in some ways an argument against it, it might be a bit daunting (though if you enjoy it its a great thing because there is so much of it to enjoy.) Its not just me either, a lot of King fans Ive spoken to over the years have it as their favourite of his books, I think he acknowledges himself (in On Writing which is part autobiography) its the one his fans rave about the most.
If not that, its hard to say. As I said I enjoyed most of the ones I read, I found him very consistent, but having read a lot as a teen I felt I had kind of grown out of it, rightly or wrongly. But you could look at a load of them, read the backs and see which concepts jump out at you, the chances are itll be very readable.
I mentioned The Long Walk - that is one of four short stories in The Bachman Books. If you dont go for The Stand because of length I would maybe recommend that, because as far as I remember all four were excellent.