I 2nd, 3rd, 4th, 5th, whatever any and all books by Phillipa Gregory. The rampant incest is just... gah. The Wideacre trilogy literally gave me nightmares for months afterward. And I don't tend to react that way to things. It was just so profoundly disturbing.
When I was in 8th grade, my town decided to start a new thing called "Read around Town" (well, the town's NAME, but you know what I mean.) So they encouraged as many people as they could to read the same book--it was all over the public library, and they made all the 8th grade English classes read it, etc. The book they chose for their very first year? Nothing but the Truth by Avi. Oh my good lord. I realize now that I can't stand books/movies/"humor" that are all based on the premise of "Somebody told a lie. Then they told more lies to cover up the lie. Then other people figured out the lie but to save face they told a lie to cover up the other person's lie. And if that person had just told the truth oh, the FIRST time they were confronted about their lie, this book would have been about 300 pages shorter." Our English teacher raffled off copies of the book once we were done reading it (after all, the next year they were going to pick a different book to "read around town"!). I won one and tore it apart, page by page.
For the same reason, I can't handle the Shopaholic series, or 99% of comedy movies today. All about lying and covering it up. The worst is when the main character never actually has to own up to their mistakes--everything just works out perfectly in the end, therefore justifying all the lying they did (and setting them up to make a sequel.)
I really disliked the third book in the Hunger Games trilogy, Mockingjay. Not going to spoil it, but when I closed that book at the end, I felt like I had just finished watching a horror movie while tripping on LSD. (Not that I've ever been on LSD, but that's how I imagine it would feel.) I loved book 1, and book 2 was pretty good, too. Book 3... no.
The Babysitter's Club series was one of my favorites as a kid, but I always skipped the introductory chapter, too. After 100 books in the main series (not to mention all the "Adventures", "Super Adventures", "Mysteries" and "Super Mysteries"), it was a little incredible to believe that they were all still "13 years old, attending Stoneybrook Middle School."