The music that accompanies this video. Though it plays in several locations, the most notable one is the Ragnaros fight in the Molten Core. It meshes so well with the tone of the battle and Rag's own quotes that it may well be WoW's first occurrence of boss music.
Most of the Wrath music would qualify, but mostly anything to do with the Lich King or Icecrown Citadel. Assault on New Avalon for the Lich King's offensive, and the dragon-slaying music that accompanies the fight against Sindragosa.
Most of Cataclysm's music does not disappoint. The Login screen music basically mixes all of the previous login screen songs into something absolutely beautiful. But that's not the best part, there is a set of 369 songs said to be in Cataclysm, some of them remakes of Vanilla World of Warcraft songs. There are bound to be many keepers in it. Some listed here. The total amount of music in WoW as of 5.4 seems to be over 40 hours.
The theme of Twilight's Hammer, probably the coolest theme for a death's cult of evil ever.
The soundtrack used on the Pandaren starting area (and reused in lots of other places for great effect) also deserves a mention here. It's also a remix of part of the Wrath of the Lich King main theme (namely, the part that also holds the Yogg-Saron theme).
An excellent piece comes at the end of the Tanaan Jungle intro, where you and your allies are running as fast as possible to the docks, with the Iron Horde in hot pursuit, and this playing in the background, with Grommash Hellscream roaring that there is no place for you to run in his world.