1. Steve McQueen
As great as Newman was, McQueen legitimately had what it took to be a top-flight professional driver. He enjoyed racing dirt bikes and competed in grueling endurance races like the Baja 1000.** More impressively, however, he won his class in the 12 Hours of Sebring, co-driving with F1 driver/Revlon heir Peter Revson, and nearly beat Mario Andretti for the overall win in the process. Legend has it that he was all set to co-drive with Sir Jackie Stewart at Le Mans, but that the studios threatened to pull all funding for his eponymous movie about the race if he competed, which is a shame.
NOTE: Paul Walker competed in several high-level amateur races alongside friend and MLB pitcher CJ Wilson, and it is with the utmost respect that he is omitted from this list.
*Newman was inducted into the Sports Car Club of America Hall of Fame in 2009.
**McQueen was inducted into the Off Road Motorsports Hall of Fame in 1978.
UPDATE: McQueen won his class at Sebring, only just losing out to Andretti for the overall victory.