It might not actually be my first choice, but this one of the few on my short list that no one else has mentioned.
I read it in my mid teens (not too long after we stopped playing Rambo in the back yard) and it profoundly affected the way I view war, violence, and suffering. I've only read it the once (almost 25 years ago), and I hardly remember the story at all, but I vividly remember the feeling that it gave me.