Megalodon is just a giant mouth; it's easy to beat it if you have a long body that can wrap around it and squeeze the life out of it, because it can't do much to get itself free. Unfortunately for Mr Shark, that's exactly what the Big Octo and Leviathan have.
So finally, it's Big Octo vs Leviathan...and Leviathan is clearly the winner. No questions. It's a fucking monster with all sorts of crazy fantasy features that allow it to basically kill anything.
i'll pick the one not extinct or fictional. Giant squid sightings off the coast of Mexico baby! Check out the episode of monster hunters, i think they actually get a crappy/half focused shot of one. sigh, never any good shots of monsters like that.