The most surprising plot twist or ending
I constantly forget how hard these questions are. Well...What's happened to Goodbye by Sarah Dessen wrapped up in a way I was both prepared for and unprepared for. It's not a hardcore plot twist though. I think I'm going to go with the Wither series because that series likely to kill people and it hurt a lot.
Definitely Hopeless by Colleen Hoover. Or, Catching Fire, when I read it the first time. I totally should have seen it coming. I just... didn't.