Fortune Magazine is out with its Fortune 500, its annual ranking of America’s largest and most powerful corporations. In pole position is Exxon, whose profits in 2011 rose 35% to $41.1 billion. A good haul, by any definition. Only Wal-Mart stood in the way of a clean sweep by oil and gas companies at the top of the leaderboard, with Chevron ($26.9 billion profit) and ConocoPhillips ($12.4 million profit ) taking spots 3 and 4.