Greece is not famous for its chocolate, but there are some very good chocolatiers here in Athens and also in Thessaloniki.
In Athens I suggest you try something from Aristokratikon. This is truly a family concern and they still use the recipes passed down from their grand father.
For cutting edge chocolates to rival anything made in Paris or London go to Sweet Alchemy. This is the brain child of Stelios Parliaros the most famous chocalatier patissier in Greece. His cakes and chocolates are divine and he makes them all himself. He only uses Valrhona chocolate in his recipes.
He made a cake for our sons birthday , chocolate and praline it was sublime.
He also publishes a magazine every month, called "Sweet Stories" devoted to all things sweet.