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?
May be you are right. North Korea will step ahead to replace their fiat with crypto currencies forever. Besides I think Japan will also increase the usage of the digital currency and may someday replace fiat with digital currencies altogether. Lets see what is waiting for us.