Cash it is most liquid thing. Cash have accepted everywhere, but it depend of the currency of cash - Usd Dollar of Euro there are most liquied, but some exotic currencies can be accepted only inside home countries.
Liquidity of fiat is one disadvantage of fiat money. for me bitcoin was the better than the paper money. First paper money was controlled by banks or even government. Bitcoin is decentralized and it is one of the major advantage of bitcoin to paper money.