Picking a "favorite" is generally hard. I once learned that the best way to phrase this question is in the form of e.g. "What are some of your favorite books?" so the responder doesn't have to force rank their all-time-favorite in their head, which can feel taxing. So I'll name two:
I'm actually a bit surprised to not see "Brave New World" mentioned yet. That was a life-changing experience to my teenage self
And since someone else has already picked Steppenwolf by Hesse, I'll mention Demian instead.
I remember getting assigned this book in high school, I think Freshman Year, and not being able to put it down. Just left me with this feeling of, "Shit, if we sacrifice freedom and individuality for happiness and a sense of security then we're really going to be fucked." I know that's probably too political for HN, but man, the message I got from this book has resonated loud and clear with me ever since.
I'm actually a bit surprised to not see "Brave New World" mentioned yet. That was a life-changing experience to my teenage self
And since someone else has already picked Steppenwolf by Hesse, I'll mention Demian instead.