This I'm not entirely sure about. You have choices like Illyria and Glory who were more powerful in their time but we've only seen them once they have been bound. The Senior Partners lack credibility since we never see them. Jasmine was a novel idea and a great actress portraying her to boot but still she simply makes everyone love her, not as scary as the 'biggest bad of them all' would be. So that narrows it down to Dark Willow and the First Evil. Both were effective but since the First Evil is non corporeal it makes it less intimidating and more creepy. So I suppose my choice is Dark Willow. Not only was she effective in carrying out her vengeance but she was near the most powerful thing ever and she was corporeal.
