I really enjoyed Mrs. Dalloway by Virginia Woolf. Not something I would usually pick up, especially since the actual plot is so boring, but I loved the writing style. Also, one of my friends read The Count of Monte Cristo for a class and loved it so much that I decided to read it. Still only halfway through, but it's great so far. I wish I read stuff like that for class.
On the opposite end of the spectrum, I hated Faulkner's Go Down, Moses. I mean, I'm okay with books I dislike if they're quick reads, and I'm okay with slow reads if I like the book otherwise, but Go Down, Moses didn't seem to be worth the time and thought I put into it. I understand why it's an important book, I just despised reading it.
Edit: Just remembered! There was a class where we read Rosencrantz and Guildenstern Are Dead and Small Gods. That was THE BEST. My favorite play and one of my favorite books by my favorite author, both in one class. I have seen the R&G movie so many times that I barely had to look at the page when we read scenes aloud. I loved that class.