Hmmmm, there are a huge number of songs I could think of that would fit this bill, and the ones I come up with off the top of my head just now probably won't be the best ones, so please bear with me:
Tori Amos -
'Icicle'
Sure, maybe this song is just Tori and a piano, but the way it whispers itself in, before crashing almightily in your face, before dying down again, only to swell once more into a total beautiful, orgasmic melodic climax, before finally fading away, SCREAMS epic to my ears.
My Dying Bride -
'A Doomed Lover'
Now, I don't think MDb are one of the best bands there has ever been, fond as I may be of them, but this song also sounds very epic to me. Very doom-laden, as it gradually swells and swells into it's noisy climax, it almost sounds like a musical interperetation of the end of the world. And for that, I think to call it epic would be fair.
Arcturus -
'Ad Astra'
Mostly instrumental, but a very well projected dark mood here. Lots of nice layers, with the guitars, keys and strings all working together to build tension throughout. Very powerful, I find.
Led Zepellin -
'No Quarter'
A brilliant record, possibly my favourite from Led Zep. Yes, it's less obviously epic than
'Kashmir', but this one (to me at least) captures a much more intense atmosphere. A stunning song, really.
Tool -
'Wings For Marie Pt. 1'/10 000 Days (Wings Pt. 2)'
These two need to be played back to back for the effect to work properly, I think. 17 minutes of tension, atmosphere and progression. By the end, there is so much power to this piece, it genuinely impresses me more every time I hear it.
Snow Patrol Feat. Martha Wainwright -
'Set Fire To The Third Bar'
Barely 3 minutes long, and still much more epic to my ears than some of the choices that I've read. Passion, angst, heartache... it's like the pain of being stuck at great distance from the one you love is condensed into one short, beautiful song. Stunning, IMHO.
Hope Of The States -
'Black Dollar Bills'
Probably my favourite song. This, to me, is the best musical representation of hope I've heard yet. So bleak, and yet so optimistic all at once, it just makes me want to cry, it's THAT joyous. The most epic song I can think of.
I might add more later, I don't have time just now
