What is the least changed language in use today?

Everything about this question makes unwarranted assumptions about how language works.

Authentic is not a valid concept in language. Also historical similarity is always a modern construction.

At best you could ask about which language had best documented links with its earlier phases in the distant past. In that case, some candidates could be Sanskrit, Modern Greek, Modern Hebrew (a great example of conscious construction), Liturgical Coptic, Mandarin. Obviously even Latin could in some respects still make this list.

Every single one of these examples is fraught with controversy and questions about identity, continuity, evidence, etc. So while it is important to ask questions about the processes driving the development of languages, words like authentic or oldest are far too normative and politically laden to be of any use.