So many books, so little time.
Sorry to start out with some musing, but what do you mean by "best"? Looking at the previous posts, I would say that they can be divided into two categories: those recommending Big Books (meaningful, thought-provoking - in short, Great Literature), and those recommending good reads (easy to swallow, gripping - and yet which in many cases still make you think). F'r instance, I would put To Kill A Mockingbird, and Jane Eyre, in the first category, and A Hitchhiker's Guide, or the Harry Potter books, in the second.
OK. So my favorite(s)? If you fall back on the old traditional question, what book would be the one book you would want to have with you if you were shipwrecked on an island, I would go for the Bible. (Or, of course, "Raftmaking for Dummies".)
If you ask which book I could and have re-read again and again, it would be the Lord of the Rings trilogy, but also for example Leo Rosten's Captain Newman, M.D. (anyone remember that one, from fifty years back?)
And if you ask what books I simply found so good, I couldn't stop reading them until the end, it would be Centennial, Shogun, the first Harry Potter, The Alchemist, and The Name of the Rose.