I've been thinking about Warren Buffett's most underrated piece of advice lately, and it's honestly something most traders completely miss while chasing the next big stock. Back at Berkshire Hathaway's 2021 shareholder meeting, he straight up said: "For most people, the best thing to do is to own the S&P 500 index fund." This isn't flashy, but it's powerful.

Here's what caught my attention: Buffett didn't just talk about it—he actually put his money where his mouth is. In the early 2000s, he made a half-million-dollar bet against some of Wall Street's best hedge fund managers. He picked the Vanguard S&P 500 ETF and went head-to-head with Protégé Partners from 2008 to 2017. The result? Buffett's index fund returned 126% while the best hedge fund only managed 88%. He won by 38 percentage points—beating over 100 financial professionals who charge clients massive fees.

What really stands out is the fee structure. While those hedge fund managers are taking cuts, the Vanguard S&P 500 ETF charges just 0.03% in expenses. That's $3 per year on every $10,000 invested. The math is almost unfair in favor of the index.

The S&P 500 itself tracks 500 large U.S. companies across all sectors—basically giving you exposure to the backbone of the American economy. Over the past 30 years, it's delivered a 1,820% total return, averaging 10.3% annually. That's through recessions, bear markets, corrections, everything.

Let me break down what this actually means for your portfolio: If you invested $450 monthly into this Warren Buffett-approved index fund, you'd be looking at roughly $87,300 after 10 years, $320,000 after 20 years, and around $940,200 after 30 years—assuming dividends get reinvested. That's the power of compounding working for you.

The top holdings in the fund read like a who's who: Nvidia at 8.4%, Apple at 6.8%, Microsoft at 6.5%, Amazon, Alphabet, and the rest. You're getting diversified exposure to the companies actually moving the global economy.

I get why people are attracted to picking individual stocks or chasing emerging opportunities. But there's something to be said for Buffett's philosophy here. The Warren Buffett index fund strategy isn't exciting, but it's consistently beaten the professionals. And honestly, that track record is hard to argue with.