Well, I am pretty certain this will change, but currently the best book I ever read would have to be River of Gods, by Ian McDonald.
http://www.bookjetty.com/books/0743256700/river-gods
At any given moment, the books that have the most impact on me are the books that I would have read recently, and hence my favorite book at any time keeps changing. I rarely ever go back to re-read any books.
However, River of Gods is the only exception I can think of. I read it a couple of years ago, but it still makes an impression :).