Question is which crypto?
Probably bitcoin, but possible an altcoin could surpass it if it has just the right features to be a currency.
First identify the 'right' features or focus, if the qualities to look for are known, then it will make identifying the potential currencies easier.
I'm not an expert, so I don't know exactly, but for us to have 1 coin to use mainly, it has to be completely perfect and sustainable for a while. Imagine if cash was designed with some weaknesses, and then they introduced a new type of currency, making your cash useless.