Bitcoin is not so strong, we can say, it is just the beginning. Even marketcap is not much more that's why many country don't have any rule for it's regulation. It's price value is always volatile in nature. We cannot compare it with any country's currency since they are stable, but bitcoin may not be.
In fact the investors can make bitcoin more stronger, but in current time we can see that bitcoin is still not so much popular and the users of bitcoin are still not in number therefor i do not think that bitcoin can be called as the most strongest currency of the word. i think it will take a good time to become the most strongest currency of the world when its users will increase more and more.