Gerald's Game was absolutely excruciating; I will not read it again. I experienced it the first time by unabridged audio, and found it a form of torture. Some time later, I read it, assuming it would be less annoying and monotonous that way, as I can read much faster than an audiobook reader can speak. Made no difference. I don't blame King's writing; the situation is the problem, and he describes everything that happens to his protagonist in detail, creating a complete vicarious experience of being handcuffed to a bed for several days. In a way, the problem with Gerald's Game may be that it's too well-written, too realistic; as painfully dull, yet nerve-wracking and frustrating as really being in that kind of isolated, trapped state.
I didn't really like The Regulators, either. I may try re-reading that one eventually, as it may have been the period of time in which I read it that created the unrelentingly grim atmosphere I associate with that book, as opposed to the novel itself. I do seem to recall finding the manifestation of the evil entity in that book rather unbelievable and absurd, but I was only 21 at the time and hadn't experienced as many unbelievable and absurd things as I have since then. At 32, I may understand the elements of satire that I suspect are present in that book much better, and I know that my own credulity, like my sense of humour, has expanded considerably since then.
Otherwise, I'm having a hard time thinking of a King novel I didn't enjoy. From what I've read above, I suspect that the things a lot of us have problems with in King's work are the same things we have trouble with in reality: injustices happen, good people die for no good reason, etc. King always says he has no control over his stories; he writes what he sees. This is why, in spite of the so-called 'paranormal' or 'supernatural' elements in them, King's novels and shorter works have a verisimilitude to them that is lacking in most fiction, a sense of reality and truth. People in King stories behave as well and as badly as people really do, and weird things happen for no reason anyone can determine or prove, without all the neat explanations that fill other fictions and are never present in real life. It's this honesty that I like about king, and that makes it difficult for me to select books I didn't like, however unpleasant or strange their contents may be. Wouldn't recommend reading the Bachman Books if you're depressed, however, particularly the early ones (though I do think everyone ought to read The Running Man, so they'll be prepared for the kind of future our current leaders are creating for us).