The ideal future of currency for me, is a free, and open interchange.
All forms of tender/exchange/currency should be usable as a method of payment 🤗.
Discussion
That doesn't answer the question of which one you consider best, of course.
I like all currencies in honesty, just the intricate designs of the currency, the culture, & values, the history.
As much as I hate on The United States Federal Reserve Notes they are quite beautiful as well, that being said I prefer cryptocurrencies (primarily Bitcoin, and Monero) but I'm open to anything, Goldbacks, Swiss Francs, Chinese Yuan, Japanese Yen, Zcash, Bitcoin Cash, etc.
Heck maybe even Zimbabwean Dollars, and Old German Duetsch Marks.