|Lament of the Highborne by Jiao Qiao|
I know, everyone's favorite NPC is Sylvanas. I'm going cliche and generic on this question, but its because she truly is my favorite. I think I've mentioned before (maybe on my old blog) how I always love the tragic hero. I almost chose Arthas as my favorite NPC, but I just couldn't do it. Afterall, he isn't really a hero.
She's strong and intelligent and resilient. She died attempting to save her people. She bested the Lich King and his mind games, freeing herself and the rest of the Forsaken. After all of this she is not numb, she can still weep over the fate of her people, but still lead with an iron fist. Despite what happened at the Wrathgate, I still love her.
Blizzard obviously loves her too, hence the beautiful music scene. Here is the official video that Blizzard released during Burning Crusade.