Warcraft I: Orcish Victory Music, especially when the massive Orc Chieftain's portrait is splayed in the background. Not to mention the Orcish narrator explaining the utter destruction of the Kingdom of Azeroth.
Warcraft II: It's called simply Human 1, but the name is the only non-epic thing about it.
Orc 1. War drums playing? Rally the grunts, it's time to go to work.
Orc 3 uses a minute and a half to build up the single greatest harpsichord face melter in video game history.
The "First" Human song does a good job of tying together the overall medieval-kingdom-feel of the faction. Be sure to hear the track at 3:28, where is the music segues into what sounds like an anthem for the Human Alliance; it really encourages you to get your army moving into battle.
From the Frozen Throne expansion, "Human 4", which sums up the state of the Alliance perfectly.
The music in Arcanist Doan's room in the Scarlet Monastery. It plays elsewhere as well (including one place in the Molten Core). It's the last few seconds of a track called "Legacy" from the main game's OST (the rest of the piece isn't used).
Some of the Burning Crusade's ambient music is subdued but spectacular, be it the haunting swells of organs and strings in Terokkar Forest, or the ominous chants of Shadowmoon Valley.
The music for Shalandis Isle took new Blood Elf players by surprise, but in a very good way. It was so popular that Blizzard has since stuck it in Night Elf inns and the Temple of the Moon in Darnassus (possibly providing the first reason to hang around the city).
A Call to Arms doesn't appear very often, but you've gotta love the epicness of the second half. And the nice lead-in from the first.
An upgraded version of it appears in the Wrath of the Lich Kinglog-in music.
The distinctive leitmotif of Illidan Stormrage, found inside the Black Temple and in various places across Outland. (Unfortunately, the best examples can only be heard by going into the game's .mpq files.) A melody of ominous strings and bitter reed instruments.
And then, there is Arthas, My Son, that song playing behind the incredible intro cinematic for WOTLK.
Silvermoon City is the only city to have separate music for day and night (note that the first two minutes on those videos are the same, being the theme that plays in Falconwing Square regardless of the time of day). It's so awesome it made it into the title theme for both expansions!
Another beautiful piece of Northrend music can be found in the found in the Grizzly Hills, a gorgeous redwood forest with a distinct old American Midwest vibe, complete with lumberjacks, trappers, beavers, grizzly bears, and Native Americans-erm, trolls.
The new Orgrimmar themefor Cataclysm is the perfect representation of the Horde.
And a new entry into the rolls of music tracks that really shouldn't be bad guy themes, Breath of Al'Akir is the sweeping theme of Skywall. Sadly, the main place players will hear this piece is the Vortex Pinnacle.
Patch 4.3 released this awesomeness. It almost becomes Autobots, Rock Out! in some places, but it's done so incredibly well that you truly feel like you're fighting against a force that seeks to annihilate the entire world.
The music from Mists of Pandaria gives off an Asian vibe while remaining true to Warcraft. And it is awesome.