Wrath of the Lich King provided top quality music and ambiance, some of the best I have heard in any video game. As a fan of TBC for raiding this was the expansion for storytelling and immersion. Mist Of Pandaria is probably a close second. I would say Vanilla is second but I do feel most of it was down to Nostalgia though. Either way those three alone are great
Here's some of my favorite WotLK tunes.
http://www.youtube.com/watch?v=5nNh7...2700F&index=11
http://www.youtube.com/watch?v=KMR9K...6CEFC741B2700F
http://www.youtube.com/watch?v=bOPbN...6CEFC741B2700F
http://www.youtube.com/watch?v=orpqC...6CEFC741B2700F
http://www.youtube.com/watch?v=447vz...6CEFC741B2700F
http://www.youtube.com/watch?v=4RAX1...6CEFC741B2700F
http://www.youtube.com/watch?v=LG1s7...6CEFC741B2700F
http://www.youtube.com/watch?v=XFI36...6CEFC741B2700F
http://www.youtube.com/watch?v=mVCkP...6CEFC741B2700F
http://www.youtube.com/watch?v=6IdtS...6CEFC741B2700F (from 1:54 especially)
http://www.youtube.com/watch?v=r2yaj...6CEFC741B2700F
http://www.youtube.com/watch?v=E_Qnt...6CEFC741B2700F
http://www.youtube.com/watch?v=fH9BC...6CEFC741B2700F
http://www.youtube.com/watch?v=NsQLu...6CEFC741B2700F
http://www.youtube.com/watch?v=3jxjW...6CEFC741B2700F (love this for the haunting ambiance in the background)
http://www.youtube.com/watch?v=ptqQJ...6CEFC741B2700F