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 the 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.

Best Technology ETF

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. (MSFT), stocks that roared higher in the 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.

Best Commodity ETFs

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.

Gold and Silver ETFs

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.

The iShares Silver Trust (SLV) is the largest and most heavily traded silver-backed ETF in the U.S., but it is pricier than its gold counterparts with an annual fee of 0.50%.

Oil ETFs

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 the following 11 consecutive months,” according to the issuer.

Best Vanguard ETF 2018

Vanguard is the second-largest U.S. ETF sponsor by assets. The Pennsylvania-based index fund giant had $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 the best Vanguard ETF a difficult task.

One Vanguard ETF that could stand out in 2018 is the Vanguard FTSE Developed Markets ETF (VEA). VEA added $17.46 billion in new assets in 2017, the third-highest total among U.S.-listed ETFs. With developed market stocks trading at steep discounts relative to the S&P 500, investors could be inclined to embrace VEA again in 2018.

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.

Best ETFs to Buy And Hold

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 upsides in a bull market. That said, low volatility stocks outperform higher volatility peers over long holding periods and USMV has trailed the S&P 500 only slightly over the past three years while being less volatile.

Speaking of volatility, mid-caps are usually less volatile than small-caps over the long haul while producing better returns than large-caps without significantly more volatility. For long-term investors, an even better way to access mid-caps is with dividends via the WisdomTree U.S. MidCap Dividend Fund (DON). DON tracks a dividend-weighted index and has offered impressive long-term out-performance of the S&P MidCap400 Index.

DON has been one of the best performing mid-cap funds, active or passive since its inception more than a decade ago.

Small-caps are also winners over the long term, but smaller stocks are historically more volatile than their large- and mid-cap peers. Investors can ameliorate that situation by embracing a potent factor combination: small size and value. Small-cap value stocks have historically delivered stellar long-term returns while being less volatile than smaller stocks without the value designation.

The iShares S&P Small-Cap 600 Value ETF (IJS) has a three-year standard deviation of 14.24%, which is well below the comparable metric on benchmark small-cap growth indexes. Small-cap value index sector exposure often include financial services, industrials, and perhaps some consumer cyclical weight.

Best International ETF

Stock picking is a tricky endeavor, which is why many active managers fail to beat their benchmarks and why many investors have gravitated to passively managed ETFs. Picking stocks in markets outside the U.S. is even more difficult.

An ideal for many investors to tap foreign markets is via a broad-based approach that combines developed and emerging markets. The iShares Core MSCI Total International Stock ETF (IXUS) does that, though it tilts more toward developed markets. IXUS delivers exposure to over 3,400 stocks at a modest annual fee of 0.11%.

“The fund's market-capitalization-weighting approach skews the portfolio toward large multinational firms,” said Morningstar. “These companies tend to be more profitable and less volatile than their smaller counterparts.”

With a standard deviation of just over 12%, IXUS is also suitable for conservative investors looking to carve out international exposure in their portfolios.

Best Dividend ETF

Dividend investors have plenty of ETFs to choose from, but many of the oldest funds in this category ascribe to one of two approaches: weighting stocks by yield or measuring components by the length of dividend increase streaks.

While a company's dividend track record can be instructive, it is not the only way to forecast payout growth. After all, what a company did last year (or five years ago) does not mean it will be repeated this year. The WisdomTree U.S. Quality Dividend Growth Fund's (DGRW) methodology makes it a compelling play for any income investor.

DGRW tracks the WisdomTree U.S. Quality Dividend Growth Index, which employs quality and growth factors.

“The growth factor ranking is based on long-term earnings growth expectations, while the quality factor ranking is based on three-year historical averages for return on equity and return on assets,” according to the issuer.

Plus, DGRW pays a monthly dividend.

Best Fidelity ETF

Fidelity Investments, known primarily for its active mutual funds and massive footprint in the 401(k) market, was a late entrant to the ETF arena, but the Boston-based company has made its presence felt with the lowest cost sector ETFs on the market as well as some smart beta products.

One of the company's best ETF offerings may be the Fidelity Quality Factor ETF (FQAL), which debuted in the third quarter of 2016. The quality factor is often durable over the long-term and its hallmarks include dividend-paying companies, firms with sound balance sheets and/or impressive cash flow generation, and wide-moat companies, among other traits.

FQAL tracks the Fidelity U.S. Quality Factor Index, which is home to less than 130 stocks, suggesting the quality designation is not easy to attain. The ETF charges 0.29% per year, which is fair among smart beta strategies and Fidelity clients can trade it without a commission.

Best Energy ETF

Seasoned energy sector investors know this sector is very much a risk/reward proposition. In the ETF space, equal-weight and other smart beta strategies can be efficacious at the sector level, but with energy, it may be best to stick with prosaic methodologies.

The Fidelity MSCI Energy Index ETF (FENY) is suitable for an array of investors. Like every other cap-weighted energy ETF on the market, FENY is heavily allocated to Exxon Mobil Corp. (XOM) and Chevron Corp. (CVX), the two largest U.S. oil companies. However, FENY holds 130 stocks, indicating it has some exposure to potentially promising smaller energy stocks and a bigger roster than the S&P 500 Energy Index.

FENY has an annual fee of just 0.084%, making it the least expensive energy ETF on the market. Like FQAL, FENY is available to Fidelity clients without a commission.

Best Financial ETF

There are nearly 40 U.S.-listed ETFs dedicated to the financial services sector, plenty of which qualify as solid options. Size alone is not a determinant of an ETF's value or alpha-delivering potential, but in the case of the Financial Select Sector SPDR (XLF), size does not hurt. As the largest ETF tracking this sector, XLF has low bid/ask spreads, a factor to consider for active traders.

Home to almost 70 stocks, XLF features companies in the diversified financial services; insurance; banks; capital markets; mortgage real estate investment trusts ("REITs"); consumer finance; and thrifts and mortgage finance industries. The largest financial companies in the U.S. are usually money center or investment banks and as such, those two industries represent nearly two-thirds of XLF's weight.

The weighted average market value of XLF's components is more than $178 billion. Annualized volatility on the fund over the past three years is 17.5%.

Best Bank ETF

There are important differences between diversified financial services ETFs, such as XLF, and bank ETFs. The former group purports to be diversified while bank ETFs try to be dedicated to bank stocks without including capital markets firms or insurance providers.

The First Trust Nasdaq Bank ETF (FTXO), which debuted in the third quarter of 2016, is new relative to other bank ETFs, but the fund employs an interesting methodology. FTXO tracks the Nasdaq U.S. Smart Banks Index, which employs growth, value, and volatility factors in its weighting scheme. Components are weighted based on their scores across those factors.

Due to the stringent natures of FTXO's smart beta weighting methodology, its lineup is small compared to well-known bank benchmarks. The First Trust ETF holds 30 stocks compared to 77 in the S&P Banks Select Industry Index.

While FTXO is a departure from traditional financial services and bank ETFs, it probably should not be paired with funds such as XLF due to large overlap among major money center bank stocks including JPMorgan Chase & Co. (JPM) and Bank of America Corp. (BAC).

FTXO's top 10 holdings all qualify as large money center or super regional banks.