In Classical Greek ἄριστος was the superlative of ἀγαθός, which means "good" (I don't think this adjective is used anymore in modern Greek) so it literally means "best". This sentence in Classical Greek would be:
Αὐτή ἡ ἄριστη κρᾶσις ἐστὶ
"This is the best mixture (of wine)"
So άριστος is not only "good", but "more than good", so "excellent".