Idomeneo
Mozart wrote three different types of opera:  Idomeneo is an opera seria (serious opera).  These were written in Italian, and are about serious, dramatic stories, often stories from Ancient Greece or Rome.  La Clemenza di Tito  is also an opera seria.  Don Giovanni  is an example of a mixture of comic and serious opera, while The Magic Flute  is a German opera in the Singspiel tradition.