Have you found yours?
How does anyone define a favourite book? for me, it would be a book that I read again and again. This is something I simply don't do, therefore I cannot have a favourite book.
A Day No Pigs Would Die by Robert Newton Peck. Tears my heart out all the time, and I gain something new every time I read it. That's what makes it a classic.
My favorite book is usually what I am currently reading. In this case, it's a thought provoking book by Carlos Ruiz Zafon, The Angel's Game.