The hardest boss for me (besides the optionalls, who I can't beat) is definitely Ansem, but he's easy; just reflect his cutters back at him and dodge roll away from submit.
Anyways, what's this talk about a new Kingdom Hearts?
One who knows nothing can understand nothing.
The heart may be weak. And sometimes, it may even give in. But I've learned that deep down, there's a light that never goes out!