Microsoft’s market capitalization peaked on December 30, 1999, reaching an intraday high of $119.94 per share. With Microsoft having documented 5,160,024,593 outstanding shares the company would have had a market capitalization of $618.89 billion on December 30.
Apple’s most recent quarterly filing listed 937,406,000 outstanding shares as of July 13, 2012, and with the company’s stock price hitting $660.73 today, its market capitalization reached $619.37 billion.
Here come the caveats. If you adjust those 1999 dollars for today’s value Microsoft would be valued at $840+ billion.
Plus, there are several government-owned petroleum companies supposedly worth a whole lot more. The Saudi Arabian oil company, Saudi Aramco, is thought to be worth several trillion dollars and the Chinese company, PetroChina, IPO’ed at over a trillion dollars. But, since they’re not public and/or accountable governments those numbers are suspect.