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 theme of Twilight's Hammer, probably the coolest theme for a death's cult of evil ever.