Portnoy's Complaint (Philip Roth) although I did force myself to read it in one day because I had to read it for a literature course and IMHO these types of books are better to get over with quickly than drag the torture out. After finishing I literally threw it across my room.
Last year I gave up on In the Name of the Rose (Umberto Eco), it was too slow and too descriptive for my tastes. I also gave up on The Murder of King Tut (James Patterson) because I got annoyed with the unneccesary chapter breaks and general lack of quality.
I also am another person that couldn't get through Stephen King's Dark Tower series. I tried several times and lose interest when reading the second book (I don't think I've finished it).