I would rank a currency by [ trading volume x price ]. I think this will be more accurate to its actual usage.
In competition with all other crypto currencies which are there in this world, bitcoin is surely the best crypto coin and that there is no other coin which is that much beneficial as bitcoin is and is the top rated crypto coin too. However, when it comes to that of other regular currencies, we cannot claim bitcoin as the top rated currency because so far, it has not been declared as the legal currency.
Basically what is a currency?
It is the fact or quality of being generally accepted or in use.
"Generally Accepted" is the key word. Since Bitcoin is not generally accepted, we could not considered it as a currency. As a medium of exchange, it is very volatile and impractical using it as a form of payment. But in terms of being stable like USD,EUR & other currencies, bitcoin is not one of them. Bitcoin has its own use and lets maximize it as a future asset. No one knows if it can be quite something.