I believe that English is an enforced popularity contest. With America and Britain being economical superpowers, many countries in the world are forced to use English as the main business language.
That may have been the case a few decades ago, but now English is used by more non-native speakers than native speakers. Business, along with tourism, etc., is conducted around the world in English every day between millions of people who are not native speakers of English. It's the first global lingua franca.
There's nothing
better about English than any other language, but at the first language in that position, it's taking on its own role beyond national/political functions. In some ways there's no need to replace it, simply because there is
any language in that position. One effect will be that English will become less and less centralized around the US and the UK. Consider India, for example, but also various other places.
As for the original question about "best" lingua franca, I don't think that has an answer. Any language could be a lingua franca, and would be adapted (e.g. with vocabulary) to fit the role, but really, there are no
linguistic properties that make one language better than another.
(Sure, you can imagine what properties a constructed language would have to offer, but the fundamental flaw is that basically no one speaks them, so you'd be starting from nothing to get it to catch on. And even if that worked, what would actually happen is that, say everyone started speaking Esperanto, it would
become a natural language through usage. It would take on the same irregularities, idiosyncrasies and other features that make natural languages "imperfect", because that's what happens, naturally, through usage. Of course there are a few things we could do to improve the practicality of languages, like making teaching resources widely available, using an orthography that is easier to learn that the irregular spelling of English or the very large character inventory of Chinese, but plenty of languages already have that in place. The fact that the "biggest" languages don't also indicates that it really isn't so crucial to the "success" of a language. Those imperfect writing systems work fine and don't actually pose a practical limitation to the usage of those languages. In the end, there's nothing to "fix".)
In short, it doesn't matter what language is the lingua franca, but given that English is in place globally, it will probably stick around for a long time, regardless of geopolitical situations. Aside from the accidents of history, it could be any other language too. There aren't any
linguistic reasons for the success of English.