It's hard to believe, looking back, that GM was once the largest corporation in the U.S. (for years) and maybe the world. Then along came GE and after that Exxon-Mobil became the largest corporation in the U.S. And now it's Walmart's turn to hold the honor. What company will be next? Apple? Facebook? Google? Surely, I would think that it would be a tech company, but don't count out the NFL or Major League Baseball, which both take in billions of dollars per year.