I didn't have a favorite book like this, but there are two things that come to mind.
First, when we were all at the Fairbanks Summer Fine Arts Camp, perhaps in the summer of 1991 or 1992, Ginger had been reading Clan of the Cave Bear, which our mothers approved of, because it was Literary. So we all read it. We were at this time collectively 11 or 12, depending on what year it actually was.
Then Gaia got her hands on the sequel, Valley of the Horses. Now, for those of you who have read this book, you are probably giggling, because you know exactly what Gaia found in there. Yes, our heroine, who had been previously raped by the Neanderthals that she'd been raised with, was tenderly courted and subsequently given Sexual Healing by a random wandering ZOMG A HUMAN LIKE ME caveman. Studly, 70s romance novel cover caveman.
We were no more than 12.
Ginger read it.
Galadriel read it.
I took one scandalized look at it and decided that all things considered, I wasn't going to be reading it at quite that time.
Somewhat later (a year or two, because I'd read the whole series up to that point by the fall of 1995, because I remember discussing it with boojum) I did finally read it. But I didn't have to hide those from Parental Disapproval.
Second, the books that I was actually forbidden from reading! In elementary school, a well-meaning person in connection with the family gave me the start of the Sweet Valley Twins series. Mama was appalled, but I liked them, which appalled her more. Dad doing dramatic readings of the worst bits did not deter me. Mama did, however, review the Sweet Valley High books, and told me in no uncertain terms that I did not get to read those books.
In middle school, the library had Sweet Valley High books on the rotating paperback racks. Our trips to the public library on Saturdays were conducted under loose parental oversight if not parental supervision; I could check out anything I wanted and there was no inspection of my choices, but I had no expectation of privacy because we all wound up checking out at approximately the same time and they would have a good chance of noticing if I'd tried to sneak anything past them. The middle school library, however, had no such oversight, and I could check out anything in there, stuff it in my backpack, and bring it home with no one the wiser.
So I did. I checked out Sweet Valley High books, one at a time, and smuggled them in my binder in the neon yellow folder. They were forbidden. I ... could kind of see why. They were trash, and I was disgusted even while I was intrigued. They really weren't that good, and I went back to my science fiction very quickly.
Third, the bonus round:
With the parental oversight of what I checked out from the library, Dad would comment on my book choice. "What are you reading this stuff for?!" he wanted to know, when I settled down behind The Monkey Wrench Gang (Shawn had recommended it, and his recommendations are usually on target for me). I huffed at him and got up to snag my share of the chips. After having eaten them and washed my hands before returning to the library book (greasy fingers! bad!) I saw Dad on the couch behind MY BOOK. MINE.
This happened several times after that, but never again so immediately.