Earlier this year, an anonymous bidder at a charity auction paid $19 million to have lunch with Warren Buffett. I have no doubt that the mealtime conversation was interesting.

Most people will never get the chance to have a face-to-face talk with Buffett. However, you don't have to sit down with the Oracle of Omaha to benefit from his investing advice. If Buffett could tell you one investment to make, this would probably be it.

Not the obvious answer

You might think that Buffett would recommend Berkshire Hathaway as the one investment to make. After all, most of Buffett's personal wealth is in the stock of the giant conglomerate. 

Berkshire has been Buffett's favorite stock to buy in recent years. Since 2016, he's led Berkshire to repurchase $63 billion of its shares. That means that Buffett has bought $9 billion more of the stock than he has of Apple and Chevron combined.

Buffett also still serves as chairman and CEO of Berkshire. Those roles make him a cheerleader of sorts for the company. It would make sense if he advised others to invest in Berkshire.

Despite all of this, though, Berkshire probably isn't the one investment that Buffett would tell you to make right now. Instead, he'd almost certainly recommend buying the Vanguard 500 Index Fund ETF (VOO 0.62%).

Behind Buffett's thinking

You only have to look to Buffett's 2013 letter to Berkshire Hathaway shareholders to gain insight into why he'd likely recommend the Vanguard 500 Index Fund ETF. He wrote back then that most investors don't know enough about the earnings power of specific businesses. Because of this, they shouldn't invest in specific stocks.

However, Buffett added that investors don't need such expertise. They don't have to pick individual winners. Instead, they can invest in a cross-section of businesses via a low-cost S&P 500 index fund

Buffett revealed that his will instructs that 90% of the cash his family receives upon his death be invested in a low-cost S&P 500 fund. (He advised that the other 10% be put in short-term government bonds.) The investing icon even made a specific suggestion to his estate's trustee: Buy a Vanguard fund.

Berkshire's portfolio includes two S&P 500 index funds: Vanguard 500 Index Fund ETF and SPDR S&P 500 ETF Trust. But Vanguard's expense ratio of 0.01% is well below the SDPR ETF's expense ratio of 0.09%. You won't find a lower-cost S&P 500 index fund than VOO.

A good bet

Buffett knows that most investors don't beat the S&P 500's performance. He even made a $1 million bet years ago that a top hedge fund manager couldn't outperform a Vanguard index fund. Buffett won the bet, with his winnings going to charity.

Investing in a fund like VOO should pay off over the long run. It certainly has done so in the past. Since its inception in 2010, the ETF has delivered an average annual return of over 13.5%. The S&P 500 itself has risen by an average of close to 10% during its lifetime. 

What would Buffett recommend to investors who are willing to do the research required to evaluate individual businesses? Probably Berkshire Hathaway. The stock has handily beaten the S&P 500 since Buffett gained control of Berkshire back in 1965. Berkshire is also similar to an index fund in that it's well diversified across multiple industries.

But without knowing your background, Buffett would almost certainly advise investing in an S&P 500 index fund like VOO. He understands that it's a good long-term bet for most people.