Most authentic. The antonym of "most unauthentic" is "most authentic." Authenticity refers to the quality of being genuine, truthful, and honest. The opposite or antonym of this trait is insincerity or fakeness. If something is unauthentic or inauthentic, it means it is not genuine or real. On the contrary, if something is authentic, it is considered to be trustworthy, credible, and true. In conclusion, using the antonym "most authentic" in place of "most unauthentic" implies that something is genuinely truthful, credible and honest.