Wow... Read all the replies so far and saw a lot of books that would be up there, but for me it would probably be:
Battlefield Earth by L. Ron Hubbard (don't judge the book by the movie, if Dianetics has a hell, John Travolta is going to its deepest darkest corner for what he did with that movie
) . By now I've probably read that book close to 50 times.
All the rest of the close contenders would be:
Almost anything written by Heinlein, Asimov, Herbert, or Card.
My favorite Heinlein book would be a tie between Moon is a Harsh Mistress, Number of the Beast, and The Past through Tomorrow.
Asimov, of course the foundation series.
Herbert, the Dune series, including the later ones his son participated in authoring.
Card, the Ender series, and his Eden books too were a pretty darn good romp.