User blog comment:AYET/Luffy alone or combined effort/@comment-7614831-20150517173312/@comment-24.235.56.170-20150519043203

The only head Law took was yours, apparently. It was 100 hearts.

Seriously though, I think a good way to differentiate the strong shichibukai from the weak is their geographical position. Doffy lives in the New World, whereas Crocodile lived in Paradise.

Presumably, Mihawk, Hancock, and Kuma are headquartered somewhere close to the Red Line, but they're also quite mobile (with Mihawk wandering as far as the East Blue). Jinbe was right at the halfway. That grouping suggests that they're all of comparable strength, with Doffy being possibly the Strongest Shichibukai in the world. (This would make sense plot-wise too, since once Luffy beats the strongest shichibukai, his next opponent would logically be a Yonko.)

And if we stick to that metric, Crocodile was the weakest shichibukai at the time of his defeat, followed by Moriah who had retreated from the New World.

Basically, the further you can encroach into Yonko territory, the stronger you are. No matter what, you have to be the strongest pirate around in order to hack it as a Shichibukai, but you can always back up if you're not strong enough where you are.