Zexion. Although he did not really appear much, I can tell he is strong. After all, illusions are hard to overcome (not many people can). Also he uses his wits not to fight. He is smart and that is why he is my favourite character in the kingdom hearts series :)
If you think of it logically, zexion can be the strongest person out of all of them because he has the power of playing with people's minds... When you verse him, it might as well be just an image because he might be playing with your head. Also illusions are very hard to overcome... not many people have that mental stregnth to even just help prevent his power from affecting you.
