From Wikipedia:
His/Her Imperial Majesty (abbreviation HIM, oral address Your Imperial Majesty) — Emperors and Empresses
His/Her Imperial and Royal Majesty (abbreviation HI&RM, oral address Your Imperial and Royal Majesty) — Emperors and Empresses who were simultaneously Kings and Queens, such as the German Emperor and Emperor of Austria
His/Her Apostolic Majesty (abbreviation HAM, oral address Your Apostolic Majesty) — the King of Hungary, usually styled Imperial Majesty or Imperial and Royal Majesty as Emperor of Austria and King of Hungary, also sometimes Imperial and Royal Apostolic Majesty
His/Her Catholic Majesty (abbreviation HCM, oral address Your Catholic Majesty) — the King of Spain
His/Her Most Faithful Majesty (abbreviation HFM, oral address Your Most Faithful Majesty) — the King of Portugal
His/Her Majesty (abbreviation HM, oral address Your Majesty) — Kings, Queens and Sultans
His/Her Imperial Highness (abbreviation HIH, oral address Your Imperial Highness) — other members of an imperial House
His/Her Imperial and Royal Highness (abbreviation HI&RH, oral address Your Imperial and Royal Highness) — Archdukes of the Habsburg family, the German Crown Prince, German Crown Princess and members of the Brazilian Imperial Family; also women with one style by birth and the other by marriage
His/Her Royal Highness (abbreviation HRH, oral address Your Royal Highness) — other members of a Royal House, reigning grand dukes, members of some grand ducal houses, some princes consort
His/Her Grand Ducal Highness (abbreviation HGDH, oral address Your Grand Ducal Highness) — junior members of some grand ducal houses
His/Her Highness (abbreviation HH, oral address, Your Highness) — reigning dukes and members of reigning ducal houses, members of some grand ducal houses, junior members of some royal houses, emirs and sheikhs, also princes or princesses of nobility in several European countries, not belonging to a royal house
His/Her Ducal Serene Highness (abbreviation HDSH, oral address, Your Ducal Serene Highness — members of some ducal houses
His/Her Serene Highness (abbreviation HSH, oral address Your Serene Highness) — sovereign or mediatized Fürst ("Prince") and his family - this is a mistranslation from German Durchlaucht, the correct form should be His/Her Serenity
His/Her Illustrious Highness (abbreviation HIllH, oral address Your Illustrious Highness) — sovereign or mediatized Count and his family - this is a mistranslation too, from German Erlaucht; it should be correctly His/Her Illustriousness
His/Her Highborn - counts, barons in several European countries, and also marquesses and viscounts in the Netherlands
His/Her Grace - peers of the Polish-Lithuanian Commonwealth.
His/Her High Well-born - knights and untitled noble persons in several European countries, and also barons in the Netherlands
His/Her Excellency (abbreviation HE, oral address Your Excellency) — Governors-General
The English style Serene Highness and even more Illustrious Highness goes back to an incorrect translation. These styles originally did not exist in English-speaking countries.
His/Her Serene Highness = German: Seine/Ihre Durchlaucht; His/Her Illustrious Highness = German: Seine/Ihre Erlaucht; Italian: Sua Illustrissima; Spanish: Su Ilustrísima