I feel like a bit of an ass for posting here, but not in the other one, but wottahey.
Tommyknockers always gets credit from me for being the book that finally convinced me that it's OK to give up on a book. I still remember it - I was in high school, stayed up late reading in bed. Closed the book. Traced the silly embossed cover with a finger. Wondered why the hell I kept reading this wretched tripe. Never opened it again.
What a feeling!
Not too much later, I gave up on Needful Things as well. I guess those two would thus be my worst King reads, as they're the ones I've been unable to finish. I'd add The Gunslinger, but I never went past page 5 of that, so it'd feel dishonest.
Oddly, I didn't mind Gerald's Game all that much. Nothing great, but I was genuinely creeped out by a couple of bits in it. Most King books are very middle o' the road for me though. I've read many, but not been particularly enthused by most. It's mainly his early novels and short stories that do it for me.