Who was the greatest of all American guitarists? You probably didn't name Gary Davis, but many of his musical contemporaries considered him without peer. Bob Dylan called Davis "one of the wizards of modern music." Bob Weir of the Grateful Dead--who took lessons with Davis--claimed his musical ability "transcended any common notion of a bluesman." And the folklorist Alan Lomax called him "one of the really great geniuses of American instrumental music." But you won't find Davis alongside blues legends Robert Johnson and Muddy Waters in the Rock and Roll Hall of Fame. Despite almost universal...