Final answer:
Wolfgang Amadeus Mozart's opera that earned the most during its initial run was 'The Magic Flute'. This opera was immediately popular due to its broad appeal, and it provided Mozart with substantial financial success before his death.
Step-by-step explanation:
The opera of Wolfgang Amadeus Mozart that made the most money during its initial run was The Magic Flute (Die Zauberflöte). This opera debuted in 1791, just months before Mozart's death, and it was a huge success in Vienna. Unlike the more aristocratic audience of Don Giovanni or The Marriage of Figaro, The Magic Flute appealed to a broader audience, including the middle class, due to its engaging mixture of fairy-tale adventure and Enlightenment philosophy, presented in the format of a Singspiel (a German opera with spoken dialogue). The Magic Flute saw numerous performances during Mozart's lifetime and provided him with significant financial gain.