This is a toughie. The ones that I like that have been important to me (such as Zen and the Art of Motorcycle Maintenance) are not necessarily ones that I'd presume should be foisted on others.
You could do worse than To Kill A Mockingbird, though. It shows human suffering and unfairness, courage and resolve, and presents characters, their dialogue, and actions in real-world ways. It's also a book that can effectively, at least in my experience, stick a hand in your guts and twist.
We had to read To Kill A Mockingbird in high school lol i loved it when the class would read books. this was one of my favorites too.