It would be hard for me to pick one book as my favorite, I have several favorite authors though, not that everything may have written is the best book that I have ever read.

Robert Heinlein to start off with Time Enough for Love is my favorite by him, but I loved most all of his books.

Tom Clancy, most of the books that he is written by himself in the fiction genre are quite excellent.

Stephen King's the Stand is an outstanding book and one of the few that I enjoyed reading by him.

Any John Norman's Gor series.

The Bible is always an interesting read, and to me it changes somewhat every time I read it, not because the book itself actually changes but because my attitudes and feelings change.

Just my input