We can not say that bitcoin is the strongest currency because of a few negative issues then the price will drop drastically, bitcoin is said to be the strongest currency if marketcap has exceeded the dollar so that all international transactions use bitcoin.
Bitcoin is better than dollar or any other currency that is why Bitcoin has the highest market value. Comparison of Bitcoin with dollar is possible and considering popular in the market but one can not get huge profit with dollar as compared to Bitcoin. This is the most common assumption Bitcoin has popularity over dollar. So I consider Bitcoin is the strongest.