I'm in the process of discovering classical music myself. As you, I have mostly concerntrated on the works of Bach, Beethoven and Mozart, but here are some other pieces that have made a great impression:
Edvard Grieg - Peer Gynt Suites, Piano Concerto in A Minor
Gustav Holst - The Planets (Sounds very "metal", especially Mars, the Bringer of War)
Franz Schubert - Winterreise, 5th Symphony, "Death and the maiden", "Rosamunde"
Antonio Vivaldi - The Four Seasons