The Oracle of Omaha has gone cold turkey on his favorite stock for at least 13 months, but has built up a 37% stake in one of America's cheapest legal monopolies.Wall Street is a stomping ground for wealth creation, and few investors know this better than Berkshire Hathaway's (BRK.A -0.40%) (BRK.B -0.35%) billionaire CEO Warren Buffett.Six decades ago, the aptly dubbed Oracle of Omaha took the lead role at Berkshire. Under his watch, Berkshire's Class A shares (BRK.A) have rallied nearly 5,940,000%, as of m ...