The ruler of New Canterlot, and, by extension, the nation of New Equestria. Since the death of Bluestar the Brilliant, who never claimed the title of king, this ruler has been a king or queen.
Monarchs of New Canterlot:
Bluestar the Brilliant, Wizard-Prince, ruled 2080-2835
Queen Ephemeris, ruled 2835-3027
King Day Star, ruled 3027-3295
Queen Meridian], ruled 3295-Present