One of the basic tenets of investing is that to earn better-than-average returns, an investment typically requires higher-than-average risk. But sometimes you come across an investment that could beat a broad-based index like the S&P 500 without a high level of risk. In his most recent letter to shareholders, Warren Buffett suggests Berkshire Hathaway (NYSE: BRK.A) (NYSE: BRK.B) is one of those investments. Buffett might be a bit biased -- practically all of his $135 billion net worth is invested in the company, where he is the chief executive officer. But his reasoning is sound, and the stock looks attractive.