I don't think I could narrow it down to just one, but my top 5 are:
IT
Eyes of the Dragon
Black House
Lisey's Story
Insomnia
Even a top 5's quite hard...and subject to change, I mean, I'd like to put Firestarter in there aswell - I think thats highly underrated. Green Mile, Shawshank and The Body are also vying for a place. And theres a few like The Shining and Salem's Lot that I haven't read for such a long time, that I'd probably want to stick them in after a reread.
Anyway - right here and now...what are yours?