For sure it is currency with the biggest fee,slowest,and most unstable,in fact it is not much a currency i see btc rather like a commodity ,virtual goods but not currency,i can buy almost nothing with btc
I even not using btc to transfer fo exchange to fiat.It is just for trading
He's asking for the currencies in general, then we can't say bitcoin is the strongest one because there are dollar, euro, chinese yuan, jpy and more currencies stronger than bitcoin.