I guess the thing is, to me, it doesn't really matter who the "strongest" or "stronger" shichibukai are.
Never in the series will you hear someone go "oh man, that kuma guy, SO WEAK" or "hancock is the weakest shichibukai" or "mihawk is the strongest shichibukai." They're all situationally strong but overall damn fearsome.
Moria once matched but lost to Kaidou and put the Strawhats in a desperate sure-lose situation that only absolute good fortune (the people whose shadows had been stolen pulling Luffy aside and handing them to him) got them out of. Crocodile apparently was able to chill in the New World and clashed with Doflamingo with neither able to get the upper hand on the other. Dofla was unable to harm Crocodile with his devil-fruit… but does that mean he's weaker than Croc? Mihawk was stopped by Joz and couldn't cut Buggy but nobody looks down on him. Hancock downright wrecked pacifista that - when combined with Dofla, brought Moria to the brink of death. All this, when the Strawhats had to go all-out to take down even one pacifista. Jimbei's salt water had an advantage against Moria -- the scale isn't binary, it changes.
People like to remember the end result but never the situation or the process. Luffy had to fight Crocodile THREE times and none of them were easy fights. If it weren't for Robin's antidote, he would have died and lost the third one too. The power scale isn't "messed up." Look at Blackbeard! Luffy put a serious hurting on him! Magellan took out his whole crew without a thought! And yet he made enough of an impression on Shanks (literally) that Shanks warned him that he was much more dangerous than he appeared. He killed Thatch, he beat Ace... and in the end, it was his crew that killed Whitebeard.
Shanks said it best himself when he said that while Ace is strong, reputation and duty get mixed up in things and can confuse the issue. Blackbeard was a no-name with no reputation and no bounty. Ace underestimated him. Croc underestimated Luffy. Moria did too.
Battle isn't certain. That's why I think that "ranking" the Shichibukai is a wasted effort. The World Government doesn't go "who can this guy beat in a fight?" when they decide who to promote, though it helps. Remember, they would've promoted Blackbeard if he brought in Luffy, but he brought them Whitebeard's 2nd Div. Commander. What matters to them is their reputation. If people think the Shichibukai CAN be beaten just like that, it defeats the entire purpose of having them there as a deterrent.