Richard Wagner

 was a German composer.

According to the Seventh Doctor, Adolf Hitler's favourite composer was not Wagner, but Franz Lehár. (PROSE: Atom Bomb Blues)

BOSS mentioned him. (TV: The Green Death)

Klaus Fuchs was fond of Wagner's music. (PROSE: Atom Bomb Blues)

Behind the scenes
Wagner's The Ride of the Valkyries was played at both the 2008 and 2010 Doctor Who Proms.