Ultimately I love them all. But I did get sick of the Cata Twilight themes (although their Tauren and Night Elf themes were amazing), and TBC didn't really add much apart from the Blood Elf Theme. So it was a toss up between the original WoW (Elwynn Forest, Mulgore, Tavern music, etc), WotLK (Stormy Peaks, Ulduar titan music, Grizzly Hills, Argent Tournament, etc) and MoP (especially the Monk themes).
In the end I chose MoP because I simply adore the White Tiger temple theme. But its very hard to choose. :S
---------- Post added 2013-05-05 at 03:01 PM ----------
One of the better things to come out of Cataclysm ;-)