Some small Caribbean islands use USD exclusively.
After their national crises, Iceland almost switched to using Canadian dollars.
The 'black market' economy in Venezueala works on USD, as it does in many parts of the world.
So long as they are established currencies with widespread use and liquidity - we don't need bitcoin.
The most 'legit' currency in the world today is probably the Euro. It's the only currency backed by real assets. The reason for this is because no European country can fully trust the other. Ergo - hard currency.
Next would be Scandinavian countries, Canada, UK etc.
The Fed is not the most trusted entity, but due to the massive and widespread adoption of the USD, they have a kind of 'seignorage' and strategic backing (Oil prices etc.) - and so it's kind of 'de facto' a very good currency.
The Yen would be up there as well but the Bank of Japan has been doing crazy things to fight deflation and so it's risky for outsiders to own.