Heaviest shit I've heard:
1. Franz Schubert, Symphony No. 8
2. Anton Bruckner, Symphonies No. 3 and 8
3. Ottorino Respighi, Simfonia Dramatica and Symphonic Poems
4. Camille Saint-Saens, Symphony No 3
5. Robert Schumann, Symphony No 3
6. Ludwig van Beethoven, Symphony No 3
7. Richard Strauss, Also Sprach Zarathustra
8. W.A. Mozart, Requiem
Do you need more?
Can't forget Brahms; Symphony No. 4 [they're all good, though], and
Ein deutsches Requiem.