Total Annihilation hands down.
It's what you'd find next to the definition of epic music.

Probably won't see one for a long time, if at all. Simulation type games aren't all that popular anymore. If they do decide to release another one, I hope it's not like MW4 because it didn't feel like Battletech for me.

Actually I started the thread just to discuss Age of Conan's soundtrack, but everyone seems to be having a good time discussing their epic/favourite soundtracks, I'd hate to be an ass and spoil the fun ;)
Anyway by far this song in the entire AOC soundtrack is the best. I know, I've mentioned my other "favourites" before, but after listening through the whole OST a few times, I have to say, this one captures the gloomy mood and feel of Cimmeria, or essentially Hyboria, in general. I recommend turning up the volume so you can hear the background instruments better
http://www.youtube.com/watch?v=WvyBplnvKWo
The song takes place when you first reach Conarch Village, in Cimmeria. The setting is atop some gentle hill areas, with streams leading to waterfalls. The sky is gloomy, according to how Robert E Howard narrates in his stories, and in the surroundings, you see barbarians huddling together in their own groups, campfires providing the only source of warmth in a gloomy place. The best part of the song is that it captures the very grim, dread-filled feeling that Howard has always wanted to bring forth in the stories. Conarch Village happens to be one of the best instances in AOC. I dislike most of the rest of the game, but Conarch will always have a place in my heart because of how well it captures the original stories

A lot of the music I listen to regularly actually comes from video game soundtracks. It's nice to see other people who appreciate the contributions a good score can make to a game. :)
Some favorites of mine:
1. The Baldur's Gate series had some really incredible music. I played the second game and the expansion, Throne of Bhaal extensively and got really hooked on the music. Here's a collection of some of the better tracks. Some really sweeping pieces, rousing battle music, even the romantic themes were good.
http://www.youtube.com/watch?v=aDYEGDrO0mU&feature=related
2. The Civilization games always needed to try and capture a lot of scope with their music. After all, it was supposed to be about shepherding a civilization from its inception all the way to modern times and beyond. That's a whole lot of years. I always thought the theme music for the fourth game did a great job of that.
http://www.youtube.com/watch?v=9ppZUcfBbeM
3. Descent: Freespace 2 had some seriously haunting music. Which was a good thing. You had a game where you were just one little bitty pilot trying to fight off an enormous alien invasion that would've wiped out humanity, and the music really brought across this feeling of loneliness and isolation while still managing to be extremely tense during all the proper nail-biting moments.
http://www.youtube.com/watch?v=T3b5L1YnK1s
http://www.youtube.com/watch?v=ohKLy59aDVw&feature=related
http://www.youtube.com/watch?v=dQvwhj17wbM&feature=related
4. Freedom Fighters had some awesome use of chorals. Very stirring music. Almost made me glad the Soviets were taking over New York. Heh.
http://www.youtube.com/watch?v=eg1__cp9698 < The Intro Movie
http://www.youtube.com/watch?v=1DKyVmhuDg0&amp;feature=channel_page < The Title Theme
5. Heroes of MIght and Magic series: Always loved the music in these games, and it's hard to pick favorites, but for some really rousing music, I'd have to say this is one of the better ones, in my opinion.
http://www.youtube.com/watch?v=sQNMA2Yni_I&amp;&amp;fmt=18
6. Homeworld: Good ambient music, but the Homeworld soundtrack makes my list specifically for one track - it can be heard here:
http://www.youtube.com/watch?v=Qb-6it5GKIg
This may just have been one of the most gut-wrenching moments I can remember in a video game, and the music was such a huge part of it. Coupled with the voices of S'jet and the Fleet Commander talking about how those cryogenic trays needed to be protected because they were ALL that was left of an entire race... just... wow.
7. Jade Empire: Good music all around, but one of my favorites was the main theme. I got the game via Steam as a Christmas present, started it up, and when the main theme started playing at the title screen, I knew I was in for a hell of a ride. I wasn't disappointed.
http://www.youtube.com/watch?v=nIXR2jU2U7U&amp;feature=PlayList&amp;p=9DDD8B2CD9248597&amp;index=15
8. Mass Effect: Yeah, another BioWare game. They do good work. :)
I was pretty much hooked from the opening cutscene when you see Shepard standing there staring out the window as Udina and Anderson discuss him/her... there's that little opening crawl describing some stuff, and then the shot of the Normandy flashing by. Cut to Shepard walking up the bridge, passing the crew at their stations on the way to the helm, and you have the theme music playing in the background... just a brilliant moment.
http://www.youtube.com/watch?v=F1fdV37gCB4&amp;feature=related
9. Planescape: Torment. So... yes. Black Isle, good. They pretty much just hit the nail on the head with all of the selections, but I think one of my favorites has got to be this one right here:
http://www.youtube.com/watch?v=g05awcTskKE&amp;feature=PlayList&amp;p=709CEACB84B8F7FF&amp;index=27
I'm pretty sure someone else mentioned the Torment soundtrack as one of their favorites, but it's worth mentioning again. It's really quite good.
10. Tomb Raider franchise: Some of the games have been a little hit or miss, but for the most part, they've been good. And when they've been good, they've been very good. My favorite pieces have always been the ones that give you that sense of wonder and awe and majesty that -should- come from exploring strange and exotic locales that nobody else has been to in hundreds of years.
The opening theme to the first game: simple and a little understated, maybe, but the use of the Gregorian-style chant just makes it for me. > http://www.youtube.com/watch?v=0zkFW3fzmGU&amp;feature=related
Anyway, there's a bunch of stuff for you guys to listen to... since this somehow became a thread for recommendations and what-not and I ended up getting preachy. Whoops. ;)

You're absolutely right, this track is stunning and it fits the game pretty well.
But many videogame reviewers forgot to mention that it is Samuel Barber's Adagio for strings so people think this music was made for the game. :/

The Wing Commander series, especially the ones composed by George 'the Fatman' Sanger, Martin Galway, as well the later series composed by George Oldziey.
Available at: http://www.wcnews.com/music/

Chrono Cross still ranks very, very high for me.

I'd say Halo and World of Warcraft both have some pretty epic songs.

I own the full Halo 2 Soundtrack, and it isn't exactly epic, but it's pretty good.

Talking about epicness... Supreme Commander!

Mentioning Jets'n'Guns in another thread, I've remembered it's soundtrack: Which IS epic! :D It's free to download, sample here:
Youtube

Aliens vs Predator soundtrack is one of my favourite!

Of course, it's Machinae Supremacy :-D

My god...That video brought back so many memories, in fact Homeworld has to be the only RTS that had me caring about the story and the fate of the the people under my command, and a lot of that has to do with how perfectly the soundtrack captured the emotion of that scene. Absolute brilliance.

Total Annihilation for me,listening to it right now.