With its tumble Monday, the bellwether ETF holding U.S. equities has given up virtually all its stock market gains year to date. SPDR S&P 500 (SPY) is lagging developed and emerging markets in 2015. But it has handily beaten them over 3-, 5- and 10-year periods. SPY invests broadly across the U.S. stock market. This diversified strategy has proved a winner for successful investing in the long term.