With a marketcap that has not reached $ 1 trillion, I do not think I can say that bitcoin is the strongest currency, this is because many countries have already reached $ 1 trillion even up to thousands of USD trillions.
I'd say it is definately the strongest currency on the planet...considering it is completely unique in the way it is designed and how it opperates, and is invulnerable to the catestrophic weakness posessed by fiat currency. It is only a matter of time before it is recognised as being the strongest currency, there is no debate as to whether or not it is, the debate is over when it will take it's place as no1 currency in terms of number of users.