I've been thinking about the shichibukai's powers and have a theory as to why they are who they are. If they're that intimidating and strong, I'm guessing they have a certain skill that makes them, well, them

I think they can all do mass damage, alone. Think about it.

Kuma
He can move at the speed of light with his fruit. Is really freaking huge, and combined with his fruits double powers, can make them disappear. One by one, he can take down entire islands within minutes, even seconds.

Mihawk
We've seen him easily use his sword to take down a massive amount of ships easily in no time.

Moria
He has the ability to move anywhere with his shadow and take over anyone's body using it, very quickly. And he can make massive armies of people with his scissors. And some zombies.

Blackbeard
He can take down entire towns in seconds and if he's careful about being hit, can destroy ability users and anything else he pretty much wants.

Flamingo
He can control anyone he wants, somehow, and I'm sure he can do it massively to have the affect of an entire group of devoted nakama beating the hell out of each other.

The only I'm not sure about Jimbei and the last one. That could break apart my theory and ruin the whole thing. But we'll see how that goes.