Orange. Big groves grow notably in the Peloponnese and Western Crete.
Besides orange juice, χυμός πορτοκάλι (chymos portokali) and fizzyorange drink, πορτοκαλάδα (portakalada), Greeks also use their oranges to make a delicious orange cake, portokalopita. The flowers are used to make anthonero
The word is derived from 'Portugal', in the far west of Europe, which in myth was the land of the Hesperides, of the twilight nymphs daughters of Atlas who (along with a dragon) guarded a golden apple tree. One of its apples was plucked by the Eris, goddess of Discord, who tossed into the wedding of Pelias and Thetis, labelled 'To the Fairest' and kicked off the world's first beauty contest and the Trojan War. Some have suggest it was really an Orange of Discord; in fact the original Greek name for all citrus species was Ἑσπεριδοειδῆ or Hesperidoeidē.
The Garden of the Hesperides, by Frederick, Lord Leighton (1892)
It's not very likely, though. Sweet oranges, like nearly all citrus fruits, are only first mentioned in China in the 3rd century BC, they only reached Europe in the 10th century, when the Moors introduced them to Spain. The golden apples were more likely, if not an apple, to have been the fruit the ancient Greeks called a 'golden apple', aka the peach.
Images by Andreas NextVoyagePL on Unsplash, PD art