Today, one of the most deadly and dangerous sharks is the Great White, which can grow to lengths of 20 feet. less than two million years go, however, we would've had the Megalodon to deal with. It is perhaps the largest predatory fish ever to exist, sporting lengths of over 50 feet, and a jaw 7 feet wide. Their individual teeth could grow to 7 inches long, and during the Renaissance, fossilized teeth were thought to be the teeth of mythical dragons! It wasn't until the late 17th century that scientists realized they were looking at a really, really big shark. (Teeth are really the only remnant that we have of these beast, as cartilage fossilizes very poorly)
So what did the Megalodon eat? In short, probably everything. It is estimated they they consumed up to 2,500lbs of food each and every day, which really doesn't seem like all that much when you consider that they have to power a muscular body that weighs over 50tons. They most likely consumed whales of the time, as well as fish, and other sea mammals. Megalodons lived in coastal areas around the world. Basically nothing fed on them.
No one is 100% sure about why the Megalodon went extinct. They dominated the seas for nearly 20 million years, and then all died out about 1.6 million years ago. Habitat changes and loss of prey may have been the culprits. Some cryptozoologists believe that the Megalodon is still out there, due to unconfirmed sightings and the fact that it's extinction is relatively recent. However, due to the size and the coastal habitats of the Megalodons, their continued existence is pretty unlikely.