For me Wizard and Glass. Out of the first 5 DT books I've read its easily the worst to me but I still like it!
Tommyknockers, Needful Things and Under the Dome. They just weren't page turners for me. Became a chore to read. There was also a tiny bit in the middle of 12/11/97 that I slowed right down on but it soon picked up again. Probably spent as long reading those middle hundred or so pages as I did the rest of the book.
I cannot finish Lisey's Story. The last time I tried was a couple of years ago. I just got it on my Kindle and I am determined to get through it this time.
For me it is From a Buick 8.
I'll take a lot of heat for this, but at least I'm being honest:
It was so difficult for me to get through The Gunslinger that I haven't wanted to read the rest of the Dark Tower series.