The competition reflects the global economic concerns hanging over the markets as well as the changing technology landscape, and it's a shift from past years.

Apple passed Exxon Mobil to become the most valuable company in the United States almost eight years ago, and after 2012 it stayed in the top spot, largely unchallenged, for six years.

Big Tech's rise and fall

The company's ascension was a precursor to Big Tech's takeover of the stock market. Investors scooped up shares of the largest tech companies, believing their sales growth was unassailable, and by the second half of 2016, Facebook, Amazon, Microsoft, Alphabet and Apple were the five most valuable companies in the S&P 500.

But investors' faith in Big Tech began to weaken last year. Apple fell hard as signs of a mature mobile phone market, a softening global economy and a trade war between China and the United States raised doubts about the company's ability to sell more iPhones.

Nearly $US450 billion was wiped off its market value — an amount greater than all but six companies in the S&P 500.

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Shares of the other tech titans fell as well. But Microsoft, Amazon and Alphabet have fared far better than Apple and the broader market over the past year, as bets by the first two — and, to a lesser extent, by Alphabet — on cloud computing drove a surge in revenue.

Microsoft and Apple, which has recovered some of its losses since it reported earnings last week, are both valued at a little more than $US820 billion. Behind them are Amazon, at $US815 billion, and Alphabet, at $US797 billion.

In fifth place is investment guru Warren Buffett's conglomerate, Berkshire Hathaway. But with a market value of $US511 billion, it is far behind.