Writer's Block: Annals of Animal Warfare
Megalodon would win. Giant and Colossal squid are a major staple of the sperm whales diet. The megalodon ATE whales. The only way the squid could win is if it attached itself to the back of the shark, quickly. The shark wouldn't be able to get it's six foot jaw span anywhere near the squid, giving it ample time to enjoy it's meal.