This stock has been a huge under-the-radar winner for Buffett and still has room to run.Everyone knows about Warren Buffett's monster Apple (NASDAQ: AAPL) investment. The huge hundred-billion-dollar winner -- which Buffett recently trimmed -- is quoted time and time again as one of Berkshire Hathaway's best investments. However, the investment has been held for fewer than 10 years, while Buffett has been investing for more than 60. Buffett has bought and held American Express (AXP 1.85%) for many years long ...