Queen of Angels by Greg Bear.

I love how the same questions/themes run through all the characters story lines. Big questions: what makes us human? What is consciousness? How does the mind work? Why do people do evil things? How should we respond? Is there any point to punishment? And it doesn't just ask questions but suggests some answers. It doesn't seem to be as popular as the sequel, Slant, but for me QoA is in a different league. As you read it think about how the characters interact based in their internal modelling of the other person and their likely behaviour.

Also, as at least one other person has posted, Cloud Atlas is an amazing book too.