Who were some all-time great fighters who seemingly came out of nowhere? Azumah Nelson comes to mind - we didn't know who he was until the Sanchez fight. Larry Holmes also probably falls into this category. Even though he was undefeated, it seemed like he was a nobody bound for nowhere until the first Shavers fight. After that fight I remember being surprised at how good Holmes was.
History is full of great fighters who had the spotlight on them from the start - Sugar Rays Robinson and Leonard, Joe Louis, Muhammad Ali, Joe Frazier, George Foreman, and Oscar De La Hoya - to name a few.
History is also full of fighters who came out of nowhere to have a great fight, or series of fights, and then fade back, or be beaten back, into obscurity - guys like Buster Douglas, Pinklon Thomas, Buster Drayton, and David Bey come to mind here.
So who were the fighters who were lucky to get an opportunity at the top level of the game, took it, and stayed there long enough to make it into the "all-time great" category?