The U.S. stock market has been on a fantastic run for the past few years, but many investors are feeling nervous. Here's the problem with timing the market: The market can move faster than you. Missing out on the best days of stock market performance can cause big damage to your portfolio in the long run. One of the best stock ETFs for long-term investors is the Vanguard S&P 500 ETF (NYSEMKT: VOO). Vanguard S&P 500 ETF (VOO): 506 stocks, 15.04% annualized returns for 15 yearsThe Vanguard S&P 500 ETF is one of the world's most popular ETFs, with more than $1 trillion of investor assets under management.