Buffett keeps buying one of his favorite stocks.It has been an up and down year for Warren Buffett's portfolio. Many of his biggest positions have been trimmed aggressively. But according to recent filings, his holding company, Berkshire Hathaway, is loading up on one of Buffett's favorite stocks. Last quarter, it boosted its position by more than $500 million.On paper, this stock has it all. It's priced at a discount to the market, offers a compelling dividend yield, and could generate impressive growth ov ...