The S&P 500 is a great long-term investment, but these look better next year.The S&P 500 is set to close out another winning year, up 15.6% through 2015, and the broad market index is showing once again why it's a smart investment.The index funds that track the S&P 500 offer exposure to 500 of the top U.S. stocks, and the fund managers regularly rebalance the index, ensuring that laggards are removed and new winners are added.However, while the S&P 500 is a great long-term investment, there are ETFs that ou ...