Norse Gods. Without question.

What about Vidar? Seriously, I'm a little disappointed that noone has mentioned him yet, but Kratos doesn't have a hope against him.

Vidar is the son of Odin, second most powerful of the gods, behind Odin and the God of silence and revenge - think of a cross between Loki and Thor and youve got Vidar.

Someone earlier mentioned that Fenrir would be a suitable enemy for Kratos; well in Ragnarok, Vidar is fated to kill Fenrir with his bare hands, literally ripping him in half. Only compounding this is the fact that Vidar is one of the only gods fated to survive Ragnarok and rule the new world.

Pretty convincing stuff.