Epic, you say? Let's see...
Well, Gustav Holst's suite
The Planets has been mentioned already, and I think that's about as epic as you can get. I would also listen to Wagner, as mentioned above, and not just the Ride of the Valkyries - there's lots more great music, even in just that one opera. If you've seen Disney's
Fantasia 2000 you've heard Respighi's
Pines of Rome (think flying whales); if not, go out and give it a listen. The final movement in particular might be the sort of thing you're looking for.
Try the Berlioz
Symphonie fantastique (it usually goes by the French title, but I suppose there's a chance you might see it as 'Fantastic Symphony' or something like that). The first three movements are a little slow, and honestly you can skip those if you want - it's the last two movements that people are familiar with, anyway.
I think someone mentioned Mahler already, and really you can't go wrong with his music, it's fantastic. That being said, it's not always like O Fortuna or Ride of the Valkyries, there's a lot of much gentler stuff - beautiful, but probably not 'epic' in that way.
In addition to
Night on Bald Mountain, you might also try the finale from Mussorgsky's
Pictures at an Exhibition, which is called 'The Great Gate of Kiev.' (If you like that, consider listening to the whole piece.)
OK, this is getting long, so just a few more suggestions. Saint-Saens'
Symphony no. 3, usually called the 'Organ Symphony,' has a great moment about halfway through the second movement, when the organ comes in by itself on a loud, sustained chord, followed by the string section and then the brass in a triumphant fanfare. (If you've ever seen the movie 'Babe,' you might recognize the theme.) The rest of it is great, too, but that moment always gets to me.
Stravinsky simply cannot be passed over on a list of intense music - his
Firebird ballet music has a lot of it, as does
Rite of Spring (alternate French titles:
L'oiseau de feu and
Le sacre du primtemps, respectively). Bear in mind that
Rite of Spring is considerably more 'modern' than
Firebird; I guess it bothers some people.
FINALLY, Dmitri Shostakovich wrote a lot of music that is not only intense, but also by turns witty, despairing, beautiful, harsh, and just about anything else you could think of. His symphonies are great stuff - some possibilities are Symphony no. 5 (probably the best-known), no. 7 (I would call this the most 'epic' one), no. 10 (the second movement and parts of the last movement are downright terrifying), and perhaps the
Festive Overture (much happier than the others). If you like chamber music at all, his string quartets are simply not to be missed. The 8th quartet in particular is worth listening to.
Unfortunately, I know next to nothing about choral music. I like John Rutter's religious choral works, but I don't know whether I'd call them 'epic.'
I hope you find something somewhere in that tangled mess of references.

Do keep us posted!