Connect the dots. By simply holding a stake in the world's best-known market barometer, you're now only guaranteed to match its usually bullish long-term performance -- an average annual gain of just under 10% per year. But you're also more likely to outperform most other investors, including the professionals.

As market-capitalization-weighted index ETFs, both VOO and VTI naturally feature a high allocation to large-cap stocks. If you want to focus more on mid- and small-cap stocks, a dedicated non-core ETF like VXF might be ideal. This ETF tracks the S&P Completion Index for a 0.06% expense ratio.

While all ETFs are different, there are two Vanguard funds I personally own and plan to continue buying for decades. An S&P 500 ETF tracks the S&P 500 ( ^GSPC 1.00%), meaning it includes all the stocks within the index. These stocks come from 500 of the largest companies in the U.S. across all corners of the market.

In this piece, we’ll check in on some of the more underrated exchange-traded fund (ETF) products that may be worth looking into as the great bull market runs for another year. Undoubtedly, it can pay dividends to go beyond index funds,