i have been delving into classical music. i have found that it shares many characteristics with metal, and its wonderful music to relax to.
One shouldn't look to classical to find metallic aspects; one needs to look to metal to find classical elements. Classical was before Metal, and it is the basis of all music. It is great of you to undertake the task of listening to this eternal music as it will awaken you from the limits of contemporary listening. Now for my recommendations:
Haydn, Mozart, Beethoven, Bach, Handel are all essentials. Contrapuntal stuff is vital. You may also want to hear some Scarlatti, Vivaldi, and Purcell; as long as you have an ear for 17th and 18th century music.
After that, move to the romantics starting with Weber, Lizst, Chopin, Schumman, Brahms, Wagner.
A bit of advice when listening to Bruckner; first listen to the First Symphony then move on. His music is very difficult to digest.
Also, advice for listening to Mahler: listen to his first Four Symphonies or the Wunderhorn
series. These set the basis for his composition style.
Of course, I'm not saying avoid all of the other pieces, but for starters...
And get a book on Music Appreciation. Get a bit of perspective first.