Bitcoin is gradually breaking through into the future, but for now we can't say it's the strongest. Regardless of current price now. In addition many other counties have not heard of bitcoin.
Bitcoin is still not the strongest currency in the world as there are still a lot of countries that doesn't even know what it is. And a lot of bitcoin users convert their coins into fiat since there are only few stores who accepts bitcoin as mode of payment. For it to become the world strongest currency, it needs to be legalized. But for me, I don't want it to be legalized. I'm contented with the current status of bitcoin now.