How rapidly has the appeal of investing in exchange-traded funds (ETFs) grown? In 2003, there were 276 ETFs available worldwide, according to Statista. Last year, that number topped 8,500. By comparison, there are fewer than 6,400 stocks traded on the two major U.S. exchanges.

If you're looking for top ETFs to buy, consider consulting two sources for ideas that might at first appear to be an odd couple: Robinhood (HOOD -1.33%) and Warren Buffett. To be sure, Buffett isn't a fan of the retail trading platform. But he and the investors who use it see eye-to-eye on at least one front. 

Robinhood provides a list of the ETFs that are the most widely held in the portfolios on its trading platform. And Buffett has money in two of the three most popular.

Robinhood's top ETFs

You probably won't be surprised that the three most popular Robinhood ETFs rank among the largest ETFs on the market. The Vanguard 500 Index Fund ETF (VOO -0.60%) is No. 1 on Robinhood. It also is the third-biggest ETF in the world based on assets under management.

As its name indicates, VOO is operated by mutual fund pioneer Vanguard. The ETF tracks the S&P 500 index, which is made up of 500 of the largest publicly traded U.S. companies. Vanguard is known for offering funds with low fees, and VOO has one of the lowest expense ratios you'll find among ETFs at only 0.03%. 

SPDR S&P 500 ETF Trust (SPY -0.59%) is the second most popular ETF on Robinhood, and the biggest on the market with more than $380 billion in assets under management. It's managed by State Street and was the first ETF listed in the U.S. back in 1993. Like VOO, SPY aims to track the performance of the S&P 500 index. Its expense ratio of 0.0945% is higher than its peer, though.

Another Vanguard ETF comes in at No. 3 on the list: the Vanguard Total Stock Market Index Fund ETF (VTI -0.59%), which also stands as the fourth-largest ETF based on assets under management. It aims to track the performance of the CRSP US Total Market Index, an index with nearly 4,000 stocks that trade in the U.S. -- from large-cap stocks that are in the S&P 500 down to small-cap and even micro-cap stocks. Like VOO, VTI's expense ratio is low at 0.03%.

Buffett's favorites

Buffet has been clear in the past that he thinks highly of index funds. The multibillionaire wrote to Berkshire Hathaway (BRK.A 0.36%) (BRK.B 0.21%) shareholders nearly 10 years ago that most retail investors would be better off putting their money in low-cost S&P 500 index funds. 

Was he just giving lip service to the idea of investing in index funds? Nope. His will advises the trustee of his estate to invest 90% of the cash his family inherits into "a very low-cost S&P 500 index fund." 

Based on this, it's a pretty good bet that Buffett would endorse the two most popular ETFs on Robinhood. He even stated in the 2013 letter to Berkshire Hathaway shareholders that his will suggests that the trustee invest specifically in a Vanguard fund. 

But there's one more way to be 100% certain that VOO and SPY are Buffett's favorite ETFs. They're the only two index funds in Berkshire Hathaway's portfolio right now. Those holdings may be quite small as percentages of the total portfolio -- but it says something that they are there.

The wisdom of crowds?

Your mother probably told you as a kid that you shouldn't always follow the crowd. This case, though, should be an exception. The popularity of VOO, SPY, and VTI on Robinhood appears to support the premise of James Surowiecki's 2004 best-selling book The Wisdom of Crowds: The many can be smarter at times than the few.

Investing in any of these three ETFs can make you a lot of money over the long run. But if you're reluctant to go along with the crowd, heed the wisdom of the expert. Buffett has stated numerous times that if retail investors own a cross-section of businesses via low-cost index funds, they are "bound to do well." Even your mother would likely agree that advice from one of the world's most successful investors is worth heeding.