What is the most valuable company in America?

It used to be Exxon Mobil. But that changed Monday when Apple’s market capitalization topped $623 billion, beating the 1999 record set by Microsoft Corp. during the tech boom. Such valuations and rankings can change quickly, however, based on multiple factors that affect how a company will perform in the future. Exxon Mobil now ranks second with a market cap of $405 billion. Microsoft ranks third with a valuation of about $260 billion.