You should definitely listen to Bach's mass in B minor and his cantatas if you like vocal work and Bach. Also listen to Mozart's Requiem.

Some other good stuff would be all the symphonies by Mozart and Beethoven. Vivaldi's four seasons.

Also try Bach's Brandenburg Concertos, and Goldberg Variations.

For opera listen to some of the overtures and highlights by Wagner (Tannhauser, Tristan & Isolde, Lohengrin).