Franz Lehár (Hungarian: "Lehár Ferenc" ; 30 April 1870 – 24 October 1948) was an Austro-Hungarian composer.
 He is mainly known for his operettas, of which the most successful and best known is "The Merry Widow" ("Die lustige Witwe").
The Count of Luxemburg (German: Der Graf von Luxemburg) is a 1957 West German musical comedy film directed by Werner Jacobs and starring Gerhard Riedmann, Renate Holm and Gunther Philipp.
 It is based on the 1909 operetta "The Count of Luxemburg" by Franz Lehár.
Victor "Vic" Schoen (March 26, 1916 – January 5, 2000) was an American bandleader, arranger, and composer whose career spanned from the 1930s until his death in 2000.
 He furnished music for some of the most successful persons in show business including Benny Goodman, Glenn Miller, Count Basie, Tommy Dorsey, Harry James, Les Brown, Woody Herman, Gene Krupa, George Shearing, Jimmie Lunceford, Ray McKinley, Benny Carter, Louis Prima, Russ Morgan, Guy Lombardo, Carmen Cavallaro, Carmen Miranda, Gordon Jenkins, Joe Venuti, Victor Young, Arthur Fiedler and the Boston Pops, and his own The Vic Schoen Orchestra.
Karl Liebknecht (] ; 13 August 1871 – 15 January 1919) was a German socialist and a co-founder with Rosa Luxemburg of the Spartacist League and the Communist Party of Germany.
 He is best known for his opposition to World War I in the Reichstag and his role in the Spartacist uprising of 1919.
 The uprising was crushed by the social democrat government and the Freikorps (paramilitary units formed of World War I veterans).
 Liebknecht and Luxemburg were killed.
Ernest Martin Joseph, OBE (8 Jan 1877 – 30 Aug 1960), architect and youth worker.
 He was a leading designer of synagogues, including the Art Deco synagogue at Sheepcote Street, Birmingham, and the classically styled synagogue in St. John's Wood Road, London.
 He was also a partner in the firm of Messrs Joseph, whose best known building is Shell Mex House between the Strand and the Thames Embankment in London.
 He was one of six children of Nathan Solomon Joseph (1834-1909), also an architect and social worker, and his wife Alice who died in his infancy.
 N. S. Joseph was brother-in-law to the chief rabbi, Hermann Adler.
Antônio Carlos Gomes (] ; Campinas, July 11, 1836 – Belém, September 16, 1896) was the first New World composer whose work was accepted by Europe.
 The only non-European who was successful as an opera composer in Italy, during the "golden age of opera", contemporary to Verdi and Puccini and the first composer of non-European lineage to be accepted into the Classic tradition of music.
Katie Moss (1881–1947) was a British singer and composer whose best known work is the popular 1911 song "The Floral Dance".
The Count of Luxembourg is an operetta in two acts with English lyrics and libretto by Basil Hood and Adrian Ross, music by Franz Lehár, based on Lehár's three-act German operetta "Der Graf von Luxemburg" which had premiered in Vienna in 1909.
 Lehár made amendments to his Viennese score to accommodate the two-act adaptation.
 He also interpolated into the score three new pieces: a waltz that he had written for a commemorative performance of "Der Graf" in Vienna; a song from his first operetta, "Wiener Frauen"; and a Russian dance from the opera "Tatjana".
Richard Arnold Moores (December 12, 1909 – April 22, 1986) was an American cartoonist whose best known work was the comic strip "Gasoline Alley", which he worked on for nearly three decades.
Johann Strauss III (25 October 1864January 9, 1939; German: "Johann Strauß III" ; also known as Johann Eduard Strauss) was an Austrian composer whose father was Eduard Strauss, whose uncles were Johann Strauss II and Josef Strauss, and whose grandfather was Johann Strauss I.
 He was unofficially entrusted with the task of upholding his family's tradition after the dissolution of the Strauss Orchestra by his father in 1901.
 His talents were not fully realised during his lifetime as musical tastes had changed in the Silver Age with more popular composers such as Franz Lehár and Oscar Straus dominating the Viennese musical scene with their operettas, although his uncle, Johann Strauss II, supervised his development as a musician, a fact disputed by Eduard Strauss.

Question: The 1909 "The Count of Luxemburg" was by a composer whose best known and most successful operetta was what?
Answer: The Merry Widow