The Oracle of Omaha continues to build a sizable position in one of Wall Street's few legal monopolies.For the better part of six decades, Berkshire Hathaway (BRK.A 0.70%) (BRK.B 0.70%) CEO Warren Buffett has been crushing Wall Street's benchmark index, the S&P 500, in the return column. As of midday on Sept. 26, he'd overseen a return for his company's Class A shares (BRK.A) that exceeds 6,080,000% on a cumulative basis over 60 years, which is about 137 times greater than the total return, including divide ...