American Express Company
Berkshire owns 151,610,700 shares of American Express, good for 13.7% ownership.
Note: Amex is part of what Warren Buffett considers Berkshire's "Big Four" investments. The other three include Coca-Cola, IBM and Wells Fargo. Berkshire increased its ownership in American Express to 13.7% from 13% in 2012.
Source: Berkshire Hathaway annual shareholder letter