For me, this is going to be an unpopular and unexpected choice, but my favorite npc with a unique voice is Ahab Wheathoof, an old Tauren rancher in bloodhoof village. Why you may ask? Because his voice actor, Ezra Chatterton wanted to create an NPC for the game, and he had brain cancer. Through the make a wish foundation, he helped create Wheathoof. He later died from the cancer on October 20, 2008. May he walk with the Earth Mother.
But who's your favorite NPC with a unique voice?