Tyler Cowen, an economics professor at George Mason University, recently received an email and used it to spark an interesting discussion on his popular “Marginal Revolution” blog. The premise was a simple one, reminiscent of fantastical playground banter, but with a grown-up financial twist.

Here it is:

“You are an investor with $10 million planning to cash out in 20 years. A genie appears and offers to send you the price of one but only one asset 20 years from now to inform your investment decisions (a stock, currency pair, commodity, equity index, etc.). What do you want to know?”

First of all, this is a good dilemma to have. Knowing any future asset price and starting with $10 million? You’d pretty much have to be a professional athlete to screw that one up.

The author of the email, one Dennis Shiraev, determined that his choice would be Bitcoin.

Cowen, however, had a different take. He dismissed Bitcoin as not being correlated with enough other facts about the future of the world to be the top pick.

“This isn’t as simple as it might seem at first,” he wrote. “You might look for the most volatile price among the liquid assets whose trading you can access. But knowing the price only 20 years out then tells you little about what is happening in the meantime. Which price 20 years out gives you the most information about the global path of prices along the way?”

His pick: the Shanghai Composite SHCOMP, +2.06% .

Dozens of readers offered up all sorts of other ideas and the reasoning behind each one. Oil CLH26, , Google GOOG, -1.66% , the euro EURUSD, -0.03% , gold US:GCH6, local real estate prices, to name a few.

One commenter claimed the obvious pick would be the long bond US:USH6. “The biggest question in the next 20 years will be ‘How does U.S. manage its entitlement promises?’ and the best answer to that question will be UST 30-year. Really no choice, actually.”

And what would your pick be?