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.
I think there are some countries which are planning to make a digital currency and make their citizens use it. I can't remember fully the country but I know I've read it here.
Also North Korea, I think will not be the first one, I think there will be others first like maybe Japan, which us kind of a high tech country that will make their first step to this digital currency, but first I think we really need to make bitcoin a digital currency that can be used in instant and free transactions.