I don't want to be that guy who just posts the SMB overworld theme, but it's really impossible to just pick one song.
That said, I'm taking this one. Nothing tells a non-vocal story in 2 minutes and 24 seconds like this track. There are so many emotions, so many movements... Anticipation, determination, strife, distress, triumph, helplessness, futility, steadfastness, I could go on, but seriously, you can picture so much to this track.
I mean, I'm surprised no one made a music video to this. You can have soldiers preparing for battle - grabbing armor, weaponry, equipping themselves, and then at 0:23 the enemies are sighted, charging down a mountainside towards an open plain...But wait! 0:34 comes along, and our hero rears up on his horse, determined to drive the enemy back! He begins his charge, and at 0:40, the camera starts to slowly pan out to show the entire battlefield. At 0:46, the two armies can be seen racing towards each other, and at 0:57, they meet with the clash of steel, and the struggle begins! Bam, 1:08 comes up, and you have the reinforcements arriving, speeding down a hillside, and 1:24 they charge down, but OHSHI- 1:30 zoom in on the big bad dude's face as things look to be going according to plan, 1:44, more badassery from the other side, with champions shield-slamming people in the face and swordplay up the wazoo... 1:54, it seems the battle is helplessly lost, but then BOOM 2:06, an even greater enemy appears, and 2:10, we THINK we're going to die, but 2:16, the trumpets sound, we heed the call, - TO BATTLE!
I could go on, or wtite this in a "serious" fashion, but you get the idea. I don't want to use the word epic, I really don't, but if it hadn't been thrown around so much, this would be what comes to mind when someone says it.
Try to paint a picture of your own while listening to this fantastic track...
Oh, and just so people know, It's by Jeremy Soule [http://en.wikipedia.org/wiki/Jeremy_Soule] of Morrowind/Oblvion soundtrack fame.
I want to put so many more tracks here, but if I can pick one, right now, it would be this. Ask me in 10 minutes and it'll probably be something from Command and Conquer or Unreal Tournament or Major Stryker or Sonic the Hedgehog 2 or Doom or Mechwarrior 2 or Earthworm Jim or Final Fantasy IX or Chrono Trigger or Sunset Riders or VVVVVV or something.