There are not really beach cities in Greece. I mean. there are cities by the sea, but if you want to go somewhere to enjoy the beaches, you need to go to an island. Try Kos, Rhodes, Paros, Myconos.
These are my favorite beaches in Greece: Elafonisi, Crete. The best beach in Greece: Elafonisi beach on the south coast of Crete. Mylopotas Beach, Ios. Plaka Beach, Naxos. Platys Gialos, Mykonos. Ornos Beach, Mykonos. Mytros Beach, Kefalonia. Paleokastritsa, Corfu. Stafylo Beach, Skopelos.