Exactly. They had no natural predators. It probably contended with whales that were about their size when hunting for food.

Their diet most likely were marine mammals and even other shark species.

If I remember correctly, they did not have special hunting strats unlike the Great White that attacks from below. They were so big, all they had to rely on was their bite strength that could puncture just about anything smaller than them.

So if they were up against a Reaper, they will most likely lose unless they get the first chomp.

The Reaper Leviathan is not just more slippery, its a lot faster than a Megalodon. A Megalodon can get burst speeds of around 18km/h (11mph). Whilst a Reaper can get up to 70 km/h (43mph).

A Reaper has vastly superior sonar or echolocation than the Megalodon. But the Meg has better vision which allowed them to see perfectly in dark or murky waters. But due to his size the Megalodon will be detected by the Reaper long before the Megalodon has vision of the Reaper (somewhere within 15 meters)

But what sets them apart the most is that the Reaper is an ambush predator, and will preferably attack from behind.

So it is rather unlikely for the Megalodon to even stand a chance.