I don't see it as "most powerful" (what's powerful is the American army and passport). I see it rather as most useful. It is widely-spoken and it is the language of academia and pretty much the internet.
It is simply the result of various historical events - any language could have been used. Had the Aztecs colonized the world, we'd all be speaking Nahault.
English language is powerful of course, but the most powerful one I think is Chinese. It's a language of the future.
It took centuries for English to displace French so I don't think Chinese (or rather the official Mandarin) is going to be the lingua franca anytime soon.