who is the authentic english speaker?

I speak authentic English.

As English today is a lingua franca ("Bridging Language"), anybody who speaks English as a first language is an "authentic" English speaker.

Authentic doesn´t have anything to do with the origin or history of a language. It just means who doesn´t "artificially" (as in: not mother tongue) speak the language, but speaks it authentically/naturally. :)

Loosely speaking, anyone who speaks any dialect of English speaks English, but that doesn't mean that they are even understandable to each other.

I actually do a lesson on this each year. I do British Slang and Australian Slang. The fact is, if you are an American speaking our regional slang, talking to an Brit or Aussie doing the same, for all practical purposes they are different languages.

Try this test if you are an American. Turn on BBC-America and watch a comedy special like Eddie Izzard. You will get lost so quickly it won't even be funny because you are not only missing the references but also the slang part of their language.

If I have many an exchange student who knew how to speak British English who was totally confused here in the US because the slangs were so totally different.

The English spoken by those who live in England - remembering that the English language is a blending of many languages (French, Germanic languages, Latin etc), so all the regional accents and dialects do, in fact, qualify as authentic English.