The Oracle of Omaha has put nearly $78 billion to work in his favorite stock in less than seven years.For six decades, Berkshire Hathaway (BRK.A 1.16%) (BRK.B 1.04%) CEO Warren Buffett has demonstrated a knack for handily outperforming Wall Street's benchmark index, the S&P 500. The annualized return of Berkshire's Class A shares (BRK.A) has practically doubled up the total annualized return of the S&P 500 since the mid-1960s, with the Oracle of Omaha overseeing a cumulative increase in BRK.A that tops 6,31 ...