I think that when the Monarchy was in power, the official title was Duke of Sparta for the Greek crown prince, the διάδοχος (diádokhos). Outside of Greece of course, as you say.
I didn't know it was an unpopular title as you say.
It says on Marie-Chantal's page on Wikipedia: Her full title is occasionally given as Her Royal Highness Marie-Chantal, Crown Princess of Greece, Princess of Denmark, Duchess of Sparta. The title Duchess of Sparta (as wife of the "Duke of Sparta", the Crown Prince) was used outside of Greece but its use is waning. The title was created by George I of Greece for his son, Constantine, overlooking one point: The Greek Constitution forbade the creation of titles outside of those already inherited within the Royal Family.