Mozart was a phenomenal combination of inconsistencies. His routine and creative genius were of the highest order, his spontaneity and finish make his music delightful alike to amateurs and musicians, but he seldom seems to take matters seriously. "Don Juan," the "Requiem," and his string quartets are exceptions, for in these he is earnest and does his genius full justice.
Beethoven gave us "Fidelio." He was equally endowed with Mozart, but was actuated in what he did by earnest, deep feeling. "Fidelio," although built on the old and now discarded lines, will only take second place (musically) when some genius arises capable of writing symphonies to supersede Beethoven's nine. In "Fidelio" we still have the string of well-defined pieces, but they are rich in harmonization and polyphony.
Weber made a great impression on opera. His audacious use of the orchestra and of modulation, opened up new fields of possibility, and there is a doubt as to whether modern German opera would have become what it is, had Weber not lived. He was gifted with an inexhaustible store of melody, was equal to all dramatic situations, however exacting, and could court popular favor without belittling his art,--a very rare quality. Weber was at first Wagner's model, and "Rienzi" and "Der Fliegende Holländer" bear a distinct Weber impress.
Meyerbeer was a German, but early adopted Italian methods. He was an excellent business man, possessed ample means, and therefore secured deserved recognition early in his career, instead of having lived almost a life of deferred hopes, as is usually the good musician's lot. Meyerbeer is melodious, and is often dramatic, but unlike Weber, sometimes belittles his art in catering to public tastes. His pageant and ballet music are the most characteristic and impressive features of his operas.
Wagner expressed contempt for Meyerbeer, but evidently recognized the grandeur of the operatic pageantry of which he was the creator. We see evidences of this phase of Meyerbeer's influence until we pass the "Lohengrin" stage.
Many other good operas were produced during the first half of this century, but as they were not potential factors in operatic evolution, I shall mention them only in passing.