Majesty

Majesty (His/Her Majesty, oral address Your Majesty) was the correct form of address used for reigning monarchs.

On Earth

In an alternate timeline, the King John Hart, having been mistakenly addressed as "Your Highness" by Rhys Williams, clarified that was correctly refered to as Majesty while Highness was used for princes. (AUDIO: The Death of Captain Jack) Queen Victoria, upon being addressed simply as "woman" by Father Angelo, asserted her form of address before shooting him dead. (TV: Tooth and Claw)

Elsewhere

On Peladon, King Kellian was refered to as "his exalted Majesty". (PROSE: Legacy, TV: The Curse of Peladon) His son, who would become King Peladon of Peladon, (TV: The Curse of Peladon) and granddaughter, the future Queen Thalira, were likewise known as "Majesty". (TV: The Monster of Peladon)

According to his wife, River Song, King Hydroflax was properly refered to as "His Infinite Majesty", though he was commonly addressed as simply "Your Majesty". After his disembodied head was destroyed by his metal body, Flemming addressed the King as "your remaining Majesty". (TV: The Husbands of River Song)