I haven’t participated in BTT in a while or done much blogging for that matter (life just got in the way) but this is a great question so here it goes.
What immediately leaps to mind is the History of the Peloponnesian War by Thucydides. I have had to read the entire thing on three separate occasions in the course of my academic career and it was torturous on all three occasions. I understand why it is an important work but not readable. Of course I am not sure that it counts because it is non-fiction (although there is some debate as to whether it should be read as history or literature).
On a more contemporary note, I have tried to read Ullysses by James Joyce on numerous occasions and haven’t been able to do it. I have every reason to believe that I should like it given my reading tastes. But that cannot count because I haven’t read it.
The other classic that leaps to mind is the Catcher in the Rye by J.D. Salinger which I did read. I just didn’t enjoy it. I wasn’t very impressed with the characters nor the writing style. I can understand if you read it as a young boy perhaps the teenage angst would resonate but when I read it as an adult woman and I just wanted to tell Holden to stop whining.
I also read the Alchemist by Paulo Coelho and the Celestine Prophesy back when they were all the rage to see what all the fuss was about. I just didn’t get it. They obviously have some sort of philosophical appeal to lots of people but they just don’t have any literary merit.