Classical composers, symphonies

As mentioned above, Shostakovich has a lot of big bold works.  His 5th Symphony is probably the most popular, and it's a doozy.  There's a great recording of the Royal Philharmonic Orchestra conducted by Charles Mackerras that I love, it's a great recording, huge dynamic range, and a phenomenal performance.  I believe this is the one, but I've long lost the original CD box so I can't remember what the cover art looks like:

https://www.amazon.com/Shostakovich-Symphony-No-Festive-Overture/dp/B000003YOH

Others to consider, especially if you're willing to wade into a bit of polytonality:

Charles Ives Symphony No. 2 (it may take a few listens for it to 'click')

Paul Hindemith's Symphonic Metamorphosis 

How about Schoenberg's Gurre Lieder! An impressive work - and sadly neglected! I remember having the Ozawa BSO version on 2 LP's! The same can be said for his Verklarte Night!

If we are including Hindemith, we should mention his most popular work: Mathis der Maler.

And since you like Mahler, you should definitely check out Shostakovich and Britten.

Kodaly, Rachmaninoff, Sibelius, Ravel, Prokofiev, Chausson, Ibert

Or how about Elgar's Enigma Variations and In the South