Whitney isnt even the hardest in her game, Id rank Morty above her.
Topic closed as I was typing an answer, but if notability werent a necessary factor IMO, the correct answers would be The General from Kaiser Knuckle, Inbachi from Dodonpachi SaiDaiOuJou, Mewtwo from Pokmon Puzzle League, and Absolute Virtue from FFXI
http://i.imgur.com/k0v0z3q.gif