Umberto Eco is probably the most turgid and difficult writer I can think of. Took me quite a while and a lot of patience to get through The Name of the Rose , but Foucault's Pendulum was even worse. But I did read them both all the way through. Afterwards wasn't quite sure why.

Thomas Hardy is one I have difficulty with. Not quite sure why, but it's an effort to get through one of his novels. I think maybe because I can never identify or have any sympathy with his characters.

Then, in a different category, is The Way of All Flesh which isn't particularly hard to read, but I absolutely detest the main character. He's such a futile, snivelling simp, or, in more classical terms, a bleating whinger. I've read it twice, and both times threw it across the room several times in disgust. Possibly Butler intended this reaction, but I doubt it.