The unabridged The Stand got me into King, that was a ~700 page book that when I wasn't reading it, I was thinking about it. That doesn't happen too often, and it was unexpected as I was just looking for something long to read while stuck at an airport.
I really liked the first few books of the Dark Tower, but for some reason I lost interest and never finished Wolves of the Calla. I'll pick it up again someday, probably start at the beginning again as The Gunslinger remains one of my all-time favorites.
The last book that I couldn't put down was The Road by McCarthy. Looking forward to 11/22/63 quite a bit.