Welcome! As a metal listener, you might enjoy some of the more extreme late-Romantic and contemporary-classical music. Try the music of Edgard Varese, especially Ionisation, Hyperprism, and Poeme Electronique; also the Mahler symphonies and Stravinsky's Rite of Spring. And Wagner has already been mentioned.

Since you already like Bach's Toccata and Fugue in d minor, you might explore some of Bach's other organ music. There's lots of it, and it's all great.

But, as has been said, there is an inexhaustible mine of great stuff.

Some names to highlight, in approximately chronological order:
Medieval period: Machaut, Ockeghem
Renaissance: Josquin, Palestrina
Baroque: Monteverdi, Corelli, Vivaldi, J.S. Bach, Handel
Classical period: C.P.E. Bach, Haydn, Mozart
Transitional: Beethoven
Romantic: Berlioz, Schumann, Wagner, Brahms, Tchaikovsky, Mahler
Contemporary: Schoenberg, Stravinsky, Varese, Shostakovich, Gorecki, Elliott Carter
(Of necessity, I've left out many fine composers to emphasize these "high lights.")