I can’t think of anything else that would prey upon them. I must say though, I think a giant squid could take the shark. It would be like a minnow with teeth to the squid.
I have to second giant squid. They prey on sperm whales, first of all. Second, they just look so king-like. Sharks are too predictable as the king of the ocean, the G.S. has the all important mystery factor, that sharks surely lack.
topser- (judging by your response) giant squid actually exist, btw. Recent evidence (video, etc) has shown some large specimens underwater, and even bigger ones are out there, judging by the dinner plate size scars left on whales by their “suckers”