Every time I get asked what my favorite book is, I get a little tongue-tied. How can you just narrow it to one? On the other hand, I'm a librarian, I ought to have a favorite book. So I've been thinking about it.
Favoriting a book is a process that takes years. If a year passes and I can't remember what reading the book felt like, then it's out. (This seems to kick out the vast majority of books that I read, mysteries and nonfiction.) Last year I read a couple of books that have potential (The Year of Fog by Michelle Richmond and Crow Lake by Mary Lawson), but I need to wait a few years and see if I keep thinking about them off and on.
Your favorite book should have the same effect on you the second time you read it. I'm a little worried that rereading The Curious Incident of the Dog in the Night-Time by Mark Haddon is going to spoil it, because I already know what happens at the end.
Then there are all the books you can't admit to loving, at least to anyone over the age of 16, which for me are the Abhorsen series by Garth Nix and How I Live Now by Meg Rosoff. Also, if you love humonguous classics like Ulysses by James Joyce, you might want to keep that to yourself (it just depends on who's asking).
The only book I've read that meets all those criteria is .... The Amazing Adventures of Kavalier and Clay by Michael Chabon. That's my answer and I'm sticking with it.