Franz Joseph

 was the Emperor of Austria when the Eighth Doctor visited Vienna with Mary Shelley in September 1873. The Doctor calls him simply "Franz Joseph".

The Eighth Doctor claimed to have met him. (AUDIO: The Silver Turk)

Alfred Stahlbaum was hoping that the performances of the Silver Turk at the Vienna Exposition would be noted by the emperor or his wife, Elisabeth. (AUDIO: The Silver Turk)

Behind the scenes
While the title "Emperor" is never mentioned in the story, the wife of Franz Joseph I is called "Empress" on multiple occasions. Franz Joseph I was Emperor of Austria, King of Hungary, Croatia and Bohemia. In some parts of his empire his royal name did not include the number, which is also never mentioned in the story.