For me it's Dracula, this book is my nemesis, I must have tried it 3 times and never got even halfway through. Something about the format of diaries and letters just doesn't do it for me and breaks up my immersion in the story. I also find Jonathan and Mina's characters to be quite bland, though admittedly I never got very far. For the supposedly quintessential novel on vampires, I have to say I was disappointed.
What book is doomed to be forever unfinished on your shelf?