Henry Purcell (1658-1695), England. He wrote one complete opera: Dido and Aeneas. He also wrote Music for the Funeral of Queen Mary: March.
Johann Sebastian Bach (1685-1750), Germany. He wrote tons of pieces ranging from organ, choral, voice, and instrumental. I am going to post the link for the majority of his works that you can pick from in my sources.
Wolfgang A. Mozart (1756-1791) Some of his most famous works include: Piano Concerto No. 24 in C minor, K. 491, the Symphony No. 40 in G minor, K. 550, and the opera Don Giovanni or Le Nozze de Figaro.
Franz Schubert (1797-1828) (He is sometimes seen in the Romantic period also.) Schubert had a huge assortment of works. He is known to take his music and add text painting (in his lieders) to make the song come alive. There are so many songs, I am going to list this link also for your personal choosing.
Felix Mendelssohn (1809–1847) known for his Violin Concerto and the Wedding March from A Midsummer Night's Dream, also wrote five symphonies including the Symphony No. 4 'Italian', and several short lyrical piano pieces or Songs without Words. (http://en.wikipedia.org/wiki/List_of_Romantic-era_composers)
Giuseppe Verdi (1813–1901), major Italian opera composer, best known for Nabucco, Rigoletto, La Traviata and Aida
Baroque: Vivaldi's "The Four Seasons"; Albinoni's "Adagio in G Minor"
Classical: Carl Philipp Emanuel Bach's "Flute Concerto in G Major"; Leopold Mozart's "Cassation in G for Orchestra and Toys"
Romantic: Beethoven's "Symphony No. 9 in D Minor"