For me it was Gerald’s Game by Stephen King. I think it’s a fantastic novel with amazing character development. The main character is extremely realistic and well written. Despite the seemingly silly premise, I was invested throughout the entire thing. The main character’s development is really what sells it for me.
That being said, I am absolutely not reading it again anytime soon. There are two scenes in particular(if you’ve the book I’m sure you know) that managed to make extremely uncomfortable, mortified, and disgusted. My biggest fear is being trapped and helpless, so this book really got to me. Fantastic book, I’m never picking it up again.
Either that or Lord of the Flies… Dear God.
The last book in the Clan of the Cave Bear series, The Land of Painted Caves. I have re-read the series multiple times, but the final book was dark and overall it ended horribly for the characters I was invested in.