American Express deserves a spot in your long-term portfolio.During the past year, Warren Buffett's Berkshire Hathaway (BRK.A) (BRK.B 0.85%) sold a lot of its top stocks -- including Apple (AAPL -1.20%) and Bank of America (BAC 2.10%) -- to boost its cash holdings to record highs. For some investors, those sales suggested Buffett was bracing for a market crash.A market pullback wouldn't be surprising, since the S&P 500 (^GSPC 0.10%) has already rallied nearly 80% during the past five years and looks histori ...