The Oracle of Omaha has long been bullish on this outstanding financial company.As of this writing, Berkshire Hathaway is now in the trillion-dollar club, joining a short list of businesses to make the cut. Warren Buffett's conglomerate fully owns multiple businesses in a range of industries, but it also has a gargantuan portfolio of public equities. Apple gets a lot of the attention. But there's a financial stock that is currently Berkshire's second-largest holding that investors need to know about. In fac ...