One of the most common survey questions, and one that has become a staple of everything from Presidential elections to college interviews, is "What is your favorite Book." This is a question that you and I might (or might not) answer honestly with a friend in a bar, but almost no one answers honestly for publication. The vast majority of the answers are public posturing, selections made to make one look bright or engaged or intellectual, and not honest answers. Presidential candidates get asked to provide their current reading list and I would bet $100 that they have staff members huddle around working on the list that portrays their candidate the best. I would be shocked if even 20% of these 50 answers at the link were honestly their favorite books.
I am not sure there is a way to get an honest answer, but if I had to ask the question, I would ask, "what books have your read more than once?"
PS - I do have to recognize Robin Williams choice of the Foundation novels and in particular his statement that the Mule was his favorite character in fiction. For those who know the books (and the Foundation is definitely on my list of books I have read more than once), the Mule is a fascinating choice for Robin Williams to have made.