Banking and financial stocks rewarded Warren Buffett's loyalty last year.

The Oracle of Omaha has long been the biggest shareholder of Wells Fargo (WFC), and now the San Francisco company represents the biggest piece of his portfolio. The value of his Wells holdings rose to $15 billion at Dec. 31, up from $10.6 billion a year earlier.

Buffett's Berkshire Hathaway (BRK/A) widened its stake in Wells Fargo by 4.3% in the fourth quarter, to roughly 440 million shares, according to the Omaha, Neb., conglomerate's latest quarterly filing with the Securities and Exchange Commission.

His piece of the red and yellow stage coach amounted to 20% of Berkshire's $75.3 billion portfolio at yearend, up from 16% of his $66.1 billion portfolio in 2011.

Wells Fargo has eclipsed Coca-Cola (KO) as Buffett's top stake, according to the filing, which was first reported by San Francisco's Business Times. Berkshire holds 400 million shares in the soft drink giant, more than in Wells Fargo, but the Coke stock fell to 19.2% of his portfolio last year.

Buffett called Wells Fargo an "exceptional" company in a letter to shareholders last year.

Berkshire also owns big stakes in American Express (AXP), BNY Mellon (BK), M&T Bancorp (MTB) and U.S. Bancorp (USB).

"Our banking system is in the best shape in recent memory," Buffett told Bloomberg in January.

His investments in Amex, BNY Mellon and M&T have appreciated (see related chart). The value of his holdings in U.S. Bancorp held steady. He sold roughly half his stake in the Minneapolis bank as its stock price rose 18% from the end of 2011 to the end of 2012.

The value of his Visa holdings fell 19%, to $236 million, as Buffett parted with roughly 1.3 million of its shares in the payments company.

He also unloaded all of his stock in Mastercard, whose shares rose 32% last year.

Buffett has a notable investment in Bank of America (BAC) that did not appear in the SEC filing. He bought $5 billion of preferred stock and warrants in B of A in 2011.

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