MSCI World Etf.
Or some one of a kind real estate. But properties can drown in the sea or be in a atomic bomb radius etc. That's why the Etf is probably the better option.
But Bitcoin would be my first choice - which wasn't available by your questions premise.
I find it interesting that this answer and @enormouspony's answer both take the approach of searching for the lowest downside rather than searching for the highest upside.
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