In your opinion, what is the best book you've ever written?
I'm reluctant to choose a "best" or "favorite" of my books because they all represent the best job I could do with the story I wanted to tell at the time I chose to write it down. Nevertheless, some of the stories do have particular emotional connections that make them perhaps more meaningful.
Saints, for instance, was a labor of love -- a story about my own people and their struggles and sacrifices more than a century ago.
Ender's Game was my first published science fiction story, and then, when I adapted it to be a novel, it became my first book to earn out its advance. Even now, the public reception it receives maintains it as the foundation of my ability to pursue my career as a writer. So naturally my feelings toward Ender are very warm!
Hart's Hope, though it is dark and bleak and my least-bought novel, still has some of my best writing and I'm very proud of it.
Lost Boys is so intensely personal and tied to my own life and my feelings about my wife and children that I can't even bear to reread it. I doubt I'll ever write something so autobiographical again.
The Alvin Maker books are the most fun I've ever had writing anything. When I work on them I let myself go more than with anything else I've ever written. As a result, my outline keeps getting tossed out the window as I follow stuff that just "comes up" while I'm writing -- but I'm still proud of the result!
All my books, however, deal with themes, tales, and moral dilemmas that I care about deeply (or I would not have written about them!) and even though there are flaws in all my books, I still stand by them all as the best I could do with that story at that time.