Your question is a good one...and hard to answer. The U.S. Fortune 500 lists the top US companies in terms of REVENUE, with Wal-Mart at the top. The Global 500 lists Wal-Mart as No. 1 as well. In terms of profit, Exxon-Mobil is number 1 in the world. But which of these...or possibly another company such as Apple has more outstanding shares? This, I have been unable to determine. My guess is that Wal-Mart would also be the winner in the share category.
I'm tempted to be very clever here and say that we could actually consider the earth to be a company, with every living thing on it being a shareholder. Of course, given the increased focus on the environment and sustainable living, this becomes particularly relevant when we think about the shareholding concept and what we do or don't do to ensure the survival of the earth as a company.