It depends of country and time.
Russia (after XVI century) and any countries in ancient world:
King - Tsar (царь), Queen - Tsaritsa (царица).
Western Europe (from medieval to modern times):
King - Korol' (король), Queen - Koroleva (королева).
Empire (any empire in the World, like Chinese, Roman, Aztec etc):
Emperor - Imperator (император), Emperatress - Imperatritsa (императрица).