Austria-Hungary
Anton Haus, commander of the Austro-Hungarian navy for part of World War I, was given the title of grand admiral (Großadmiral) in 1916. No other active-duty officer (except members of the Imperial family) was ever given this rank (although Haus's immediate successor, Maximilian Njegovan, was promoted to grand admiral on the retired list in 1918).
Read more about this topic: Grand Admiral