I liked the kilrathi theme in wc2.
But now my three favourites are :
WC4 - Confronting Tolwyn, War Averted, Tolwyn's Death
WC3 - Intro
WC2 - Cutscene 1
By the way I remember the first time I saw the WC2 Intro (where we first hear the Kilrathi theme). Wahoo, it really was like a movie ! Now all games have an intro, but I think WC2 was the first one. Am I right ?