It'd be too easy to say the Twilight series, wouldn't it? I think it's the world's most mocked and hated book series to date. Why it's praised and gets recognition when it advocates horrible messages to young girls and promotes terrible writing and endorses batshit fans and their lunacy, I will never understand. It's like a plague of stupidity that I hope will die out very soon.
Besides that, for a book that I actually have read I would have to say Cut by Patricia McCormick. I remember grabbing this book from the young adults section in the bookstore some odd years ago and thought it sounded interesting. It was the biggest waste of my time reading it. The characters were so contrived and stereotypical, it was almost insulting. It dealt with self-harm but it seemed like the author was trying too hard to make it relatable to anyone that it just fell utterly flat, especially when bringing in one character that was so cliched I was cringing. I actually wanted to throw away the book because I was kind of offended.
Other than those, there aren't many books that I have read that I ended up hating entirely. I might dislike certain parts, end up feeling disappointed, but never full-on hate.