Hell march from Red alert. Obviously.

https://www.youtube.com/watch?v=yGgnywWO6Ts

Edit: changed the URL, for some reason it forced the video to the 11 minute mark

Although I am not a fan of the series because I don't like shooters that much, the following piece of music is a true masterpiece – good to listen to, easy to remember and, for musicians, a hell lot of fun to play too! I promise: Everyone knows it. :)

The original:
http://www.youtube.com/watch?v=vg0Tmydj29M

Improved in any aspect:
http://www.youtube.com/watch?v=hOB4q01VCVg

I heard those guys live when I was on vacation in the US. It was simply awesome!
http://www.youtube.com/watch?v=Op7MGTvsxHg
Definetly also check out their version of Castlevania. Do it! Now! :)

This, hands down.
Runner-Up
Also good.

Final Fantasy IV Boss Fight:
https://www.youtube.com/watch?v=6C6iHlBN_ZM

This is a battle theme.

This one respect the title of the thread but not the OP rules so no voting.

March of Cambreadth

When I need a battle soundtrack I put this on repeat.

This song is so bad ass, if enough people in power listen to this song at the same time, World War 3 might start.

MechWarrior 2: Probably one of the best soundtracks for battle.
eXceed 3rd: Jade Penetrate Black Package: If you like rocking out on your battles, then this is for you.

We will We will ROCK YOU

Is it cheating linking anything from Castlevania?
http://www.youtube.com/watch?v=8aIX-rt1scc (OoE, Albus's fight)

edit- nice metal cover,
http://www.youtube.com/watch?v=vMJDbUE7hQk

My favourite game soundtrack of all time also doubles as what I believe to be the best battle soundtrack of all time as well. The original release of the game rendered the music via synthesis on the local system/sound card so the quality of the audio varied from computer to computer but the music was epic. A year or two later when the re-packaged the game and re-released it they also updated the in-game soundtrack to full blown CD digital audio which could be played in a standard CD player as well, and they used high end synthesizers to render the music making it sound far better than any of the FM or wavetable sounds of the original game release. To date I believe this is still one of the most epic video game soundtracks ever:

<span class="bold">Warcraft II Complete Soundtrack</span>

Oh it's not just game soundtracks, is it?

Well all right, my other two would be:
This thing from Iron Sky
Preliator by Globus

Turanic Rairders - Homeworld

Bounty Hunter - Advent Rising
Armored Prayer - Gears of War 2
Brutal Battle - Total Annihilation

E: I obviously can't count to 3...

One of my favourite tracks from Advent Rising along with and [url=https://www.youtube.com/watch?v=B-cgFP3rRW4]Greater Lights.