North Korea because their fiat is already worthless and cryptocurrencies are only used by criminals like Kim Hong Un. Nah but seriously, probably a smaller country. Wasn't there a tiny island that's currency is bitcoins but it means nothing because the island's only inhabited by one family?
There are many islands that have good standards of living and that have very low populations, however many of those islands use the dollar as their currency so maybe when the dollar begins to get more unstable as the economy of the United States worsen then that could be the time that some of those small countries decide to accept bitcoin as their official currency.