Prophecies: Either the theme song (brings back so many memories of the beautiful character screen...) or that song in the Borlis Pass cinematic when you meet up with Rurik and Jalis. Very beautiful, one of the more emotional songs in the game. Third favorite has to be that song in the Shiverpeaks, the sad one with the violin - it's recycled in GW:EN too, which makes me happy. OH! And I hope everyone here has the Battle Pak for Sorrow's Furnace as those pieces are epic.
Factions: The Kaineng City music is pretty nice, very flowing and peaceful sounding.
Nightfall: Loved most of the music in this game. Favorites are probably that really hardcore-sounding battle song (the one with the war drums and stuff, and you don't hear it in the game at all until Consulate Docks and beyond), the Abaddon battle theme, and the Tihark Orchard theme, better known as the Rollerbeetle Racing song!
Eye of the North: The theme song again, it's like Prophecies but a lot more adventurous and serious sounding. The Asura areas have some nice sad-sounding pieces. Other than that I wasn't too big on the music, most of it was very plain (most of the Norn music).