Moria's ultimate power has nothing to do with shadows asgard… it is his control over shadows. Thats called Shadow Revolution, that move has the godly potential, ... deforming people and structures by invading their shadows... His power has nothing to do with zombies in the first place...
Damn you Oda, i hate the way you make people have to think twice to see Moria's power... But hell, i know the same people were calling Crocodile weak before he had his comeback.... A few pictures seem to tell more than a thousand thoughts, even if they are shown in the manga.
Power is something very fluid, the title Shichibukai is not saying anything about the power, but the ability to even out the Yonkou side of the world powers. All seven pirates are wardeciding on their own. People like Magellan and Cipher Pol. 9 are not included in this, its just 4 Yonkou vs Marine Headquaters + Shichibukai.
By now it should be clear, that "top tier" discussions are pointless and show that someone didn't understand Oda's OP-verse. Crocodile, Moria, Kuma, Jinbei and Doflamingo have demonstrated, that they fit their role and that they take their job serious. And without a doubt they have the power to do their job.