The following State and Government officials receive the style "The Most Excellent":
The style "His Excellency", which has a higher connotation than "The Most Excellent", is instead reserved for the children of an Infante or Infanta, who have the rank (but not the title) of Grandees.2
During Francoist Spain, General Francisco Franco was the de facto Dictator of Spain and properly adopted the style His Excellency, since he was both Chief of State and Government, without being a "royal".
Following the tradition from Spain, Hispanic countries adopted the styles "His Excellency" and "The Most Excellent" although they are informally used most of the time without following rules of style.
Properly used, the style "His Excellency" (or simply "Excellency") is reserved for Chiefs of State and/or Government in Republics, i.e. the President and Vice-President of the Republic. Also, though informally, this style applies for the "President of Congress" (or equivalent) and the "President of the Supreme Courts" (or equivalent). Former Presidents and Vice-Presidents of Republics usually retain the style "His/Her Excellency" after finishing their terms, as an honorific.
The style "The Most Excellent" (Excelentísimo Señor/a) applies to high-ranking officials of Republican countries that are not Chiefs of State or Government, i.e. a Minister, a Governor, an elected official.
Ambassadors of Foreign Countries also receive the style "The Most Excellent", although informally they are addressed as "Excellency".
In other countries, "His Excellency" and "The Most Excellent" are rarely used. San Marino’s Captain Regents are styled as "The Most Excellent".3
In the United States, albeit rarely, the President of the United States is styled "Excellency". The more British-style "The Honorable" is preferred for Senators, Representatives and other elected officials.
In European monarchies or former monarchies, the style "His Excellency" is rarely used. "The Most Excellent" is sometimes given to members of the minor nobility, i.e. Viscounts and/or Barons.
With the exception of the Catalan-speaking communities which use Molt Honorable Senyor(a) (Spanish: Muy Honorable Señor(a)). /wiki/Catalan_language ↩
Royal Decree 1368/1987, dated 6 November, regulating titles, forms of address and honours pertaining to the Royal Family and to the Regents. Boletín Oficial del Estado (BOE). http://www.boe.es/diario_boe/txt.php?id=BOE-A-1987-25284 ↩
Statement by The Most Excellent Captains Regent of the Republic of San Marino Mrs. Fausta Simona Morganti and Mr. Cesare Antonio Gasperoni At the High Level Plenary Meeting of the 60th Session of the General Assembly of the United Nations (PDF). 2005. https://www.un.org/webcast/summit2005/statements15/san050915eng.pdf ↩