As of April 2018, there are nearly 2,200 exchange traded products (ETPs) trading in the U.S., including almost 1,900 exchange traded funds (ETFs). So investors face a dizzying array of choices when it comes to identifying the best ETFs. Of course, what defines a “best ETF” is not static. Some investment ideas that work for 25-year-old investors are probably not appropriate for retirees. That is to say what defines a best ETF for various investors depends on individual levels of risk tolerance.

While the ETF landscape is big and growing by the day, some funds can be considered the best in their respective categories, and have the potential to maintain those perches over the long term. Here is a comprehensive list of ETFs that are among the best funds across a wide array of asset classes.

Technology is the largest sector weight in the S&P 500 and as such is often one of the largest sector exposures in a wide variety of broad market funds. Some tech funds are also among the most popular sector funds. However, traditional tech ETFs are cap-weighted, meaning the largest tech stocks take on the largest weights within these funds.

Historical data suggest other strategies can also reward investors. For example, the PowerShares S&P 500 Equal Weight Technology Portfolio (RYT) is an equal-weight fund where none of its holdings exceed weights of 1.60%. Trimming the weights of tech titans such as Apple Inc. (AAPL) and Microsoft Corp. U.S. bull market that started in March 2009, may seem risky, but RYT suggests otherwise. On the tenth anniversary of that bull market, RYT had a cumulative gain of 640.1% compared to 597.6% for the Nasdaq 100 Index.

Although RYT's equal-weight strategy means it assigns more importance to smaller stocks, the fund was only slightly more volatile than the large-cap Nasdaq 100 over those 10 years. Commodities are one of the asset classes made more accessible by ETFs. Some commodities ETFs, including gold funds, are so large that the ETF market now plays an important role in price discovery for some commodities. Among ETFs backed by physical holdings of gold, of which dozens are listed worldwide, the premier choice for professional traders is the SPDR Gold Shares (GLD) because of its robust liquidity and tight bid/ask spreads.

For buy-and-hold investors, the iShares Gold Trust (IAU) is ideal because its annual expense ratio is 15 basis points below GLD's. Oil ETPs are different animals than their precious metals counterparts. Due to the use of futures contracts, many oil ETPs make for poor long-term investments because they can expose investors to contango by using front-month futures.

The United States 12 Month Oil Fund (USL) takes a different approach. “USL's Benchmark is the near month futures contract to expire and the contracts for the following 11 months, for a total of 12 consecutive months. If the near month futures contract is within two weeks of expiration, the Benchmark will be the next month contract to expire and the contracts for following 11 consecutive months,” according to the issuer.

Vanguard is the second-largest U.S. ETF sponsor by assets. 869.60 billion in U.S. ETF assets under management as of April 11, 2018, trailing only iShares. Compared to rivals such as iShares and State Street's SPDR brand, Vanguard's total ETF stable is relatively small, but still large enough to make pinning down a best Vanguard ETF a difficult task.

One Vanguard ETF that could stand out in 2018 is the Vanguard FTSE Developed Markets ETF (VEA). 17.46 billion in new assets in 2017, the third highest total among U.S.-listed ETFs. VEA provides exposure to nearly 3,900 stocks from 24 countries. The ETF charges just 0.07% per year, making it cheaper than 93% of competing strategies. Determining the best ETFs for buy-and-hold is subjective and a matter of personal preference, but some funds could be cornerstones of almost any portfolio.

For investors looking for a buy-and-hold option for domestic large-caps, a good place to start is focusing on funds with low volatility, such as the iShares Edge MSCI Min Vol USA ETF (USMV). Low volatility ETFs, one of the dominant types in the smart beta segment, are designed to perform less poorly than traditional funds during bear markets, not capture all of the upside in a bull market.