The old
Greek language spoken way back in
Plato's time. Since the
16th century Modern Greek is spoken by virtually everyone in
Greece.
Phonetically, Classic
Greek would sound rather
alien to contemporary
Greeks, but don't ever say this to them! It is an issue that most
Greeks, even educated ones, ignore. It is probably because the
Greek alpahabet has remained the same, so
Greeks can read classic texts with no trouble (pronouncing in
Modern Greek). After all, it all looks
Greek to them! If any (non-Greek) scholar attempts to pronounce classic text in the reconstructed pronunciation, that, to Greeks is tantamount to sacrilege.
Probably nobody can reproduce exactly the Classic Greek pronunciation, not only because as native speakers of this or that language we necessarily carry over our native phonology, but also because the Classic Greek pronunciation used pitch to differentiate vowels in words, while all modern European languages (including Modern Greek) use stress instead.