The colossal squid takes some beating... we have never seen one alive, presumably they live too deep to be spotted, but dead ones have washed up or been [cauight accidentally]. On hooks laid out for other fish. Up to 14m long and half a ton in weight they are bigger and meaner than the 'Giant' squid, big enough to prey on sperm whales, which we know because whales bearing gnarly scars from their beaks/hooks have been recorded.