@Captain:
Mihawk: Fucked Krieg's entire fleet up to kill some time, took out Zoro with a toy knife, which, to him, was overkill. He's also confirmed to be the top swordsman in all the world; so, if we see a swordsman from then on out, it's implied that Mihawk is better than that.
Well destroying such a weak fleet really isn't that impressive, he produced a power scale equivalent to T-bone so it doesn't show awesomeness, just that he's capable of flying blade techniques. And defeating zoro then really isn't that greater deal, he still hadn't learnt half the moves he has, or learnt to cut metal so its not that much of a challenge, maybe if he countered some of his higher skilled abilities that he is currently showing with that toothpick it may have been impressive, I think the most impressive information so far is that mihawk and one of the Yonkou(shanks) have fought as rivals, the fact that he alone traverses the sea without the aid of a crew, and that he does whatever he wants without much concern about the government. The only effort he has ever made seems to have stemmed from his own amusement and not duty, this 'do what I want, when I want' attitude speaks volumes about this character.
Crocodile: Highest bounty we, the viewers, had seen thus far, by a large margin. Fatally dessicates an entire pirate crew in seconds, and appears bored with it. Appears invincible, as he casually survives being seemingly decapitated. We learn that if his hand touches you, it means instant death, or if he wants you to suffer, being withered alive. He fights Luffy. He completely and totally annihilates Luffy in what remains one of Luffy's biggest shut-out losses to date (only Aokiji and Kuma have ever beaten him that bad since then). Oh, and he can wipe out an entire town with one of the biggest attacks we've seen yet, and all he needs is an open hand and a tailwind.
He only stops other pirates to distance himself from any possibility of being the mastermind of baroque works, because by doing so he appears to be a hero, not the true villain, it was more of a chore then actual willingness to help. OK now we have to make something clear, Crocodile was in a sand country, his moves were available because he was surrounded by sand, for me he is strong but he was in his own element, its like Fighting Aokiji on a glacier. He had a massive advantage, yet still get beaten, to put it more simply whenever he was on the sand dunes he won, but when he fought in an underground crypt without as many moves available he was beaten, this should prove that he isn't that greater Shichibukai because he is only real competition when he is in his element. I would however say that he appears to be seriously smart, after all if his second in command hadn't betrayed him then maybe his plans would have come to fruition?
Blackbeard: Has control of a little force of nature known as gravity. Throws an entire town at Ace just to show off, and blasts him with an island-sized blast of his Darkness power. While he may have horrible defense because of his power, he also has the ability to turn off Devil fruit powers if he grabs you.
Personally I like Blackbeard, he is the grittiest shichibukai we have seen, and despite his appearance he is probably the biggest threat. First of all he has planned his future and bided his time till he was able to reach his current position, he betrayed Whitebeard and defeated two of his division commanders, and on top of this his crew seems to be something rival to Luffys. And what I really love about him is his choice of fruit, basically when he fights he wants it close and personal, he wont use some technique or ability to hurt you but his RAW BRUTE strength, this really does show he has faith in his abilities. I think his abilities to absorb towns is rather lame in comparison to his style of fighting, destroying a town isn't that impressive any more, we have seen this kind of strength with others already, but to be able to physically beat up any opponent without any worry of losing just reeks of awesomeness, I think he is one of the few people who rely on the strength more than the ability, this is my personal opinion but people like Blackbeard, Lucci, etc. don't just rely on DF abilities. to put it more plainly if you are still amazingly strong despite having your DF powers taken away then you deserve some respect.
Gekko Moria: Bounty is higher than Luffy's, making for a holy shit moment. Revealed that the entire zombie army of Thriller Bark is a product of his fruit power. Eludes Luffy at every turn with his crazy shadow powers, and reveals himself to be based more around manipulation than raw power (and quite good at it). And just to give him that "he can fuck you up anyway" factor, should he steal your shadow, sunlight can kill you. And it's not hard for him to steal your shadow, should you be casting one.
I personally like Moria and his abilities, it has so much potential but is wasted on the fact that Moria cannot be bothered to get off his arse and get his hands dirty. We saw only a fraction of what that fruit could do and yet he still proved a challenge to the crew. being able to switch places with Doppelman and control shadows really is a DF ability to be reckoned with. Its a shame such a great fruit is wasted on him, after all we saw what physical strength you can gain, and how your abilities can increase so much. It was lucky the crew was needed for the shadows otherwise they could have easily been thrown into the sea. And again if maybe Moria wasn't so greedy in absorbing shadows he could have proved a much better match, but his consciousness was slipping and his control began wavering leading to his defeat.
Bartholomew Kuma: Fucks up the entire Straw Hat crew, reveals himself to be a nigh-indestructible cyborg, and shows off what is arguably the most powerful non-logia DF to date. Adding to all this, he teleports completely instantly, and "nukes" an entire island with air. He's fought twice, and he's wiped out the entire cast of heroes both times.
I really cant argue with this, Kuma is an intelligent, strong shichibukai member, he appears to have connections with the strongest people in the world and has a reputation that isn't overshadowed by many. I think he uses his abilities extremely well, but doesn't let his strength cloud his judgements, which is something the other shichibukai do too often. There is a lot to speculate about with kuma but nothing that is concrete, for instance he appears to save the crew, but almost kills zoro, he appears to know dragon yet attack his son, and to boot appears to betray the government by cooperating with raleigh. So he really does leave one big question mark?? But the facts we can be sure of is that he is strong, has a great working knowledge of his powers, and despite his awesome strength is still a yet to be completed super weapon, leaving room for a more powerful version later.
Boa Hancock: Can cause people to instantly fall in love with her…. Usually (the initial lust didn't affect Momonga). Can turn people to stone if they have wicked thoughts about her... Which can be counteracted if you distract yourself... So in the first chapter, this Warlord already has a huge, gimping weakness in her power. This, while the others pretty much owned face in their first show-offs. And her power doesn't affect Luffy at all. Mere chapters after her introduction into the storyline, she's gimped. The only other Warlord with a weakness that stops their power, thus far, has been Crocodile. And we already know he can circumvent that weakness. And to add icing to this shit-cake, she gets the lowest bounty of the entire group (not counting Blackbeard), after we've already seen guys with bounties exceeding 300 million. At least Croc's was big for it's time in the story.
I think its too early to take such a bad view of Hancock, as she hasn't revealed much. But what we do know is she beat Luffy with one move, has
Haoushoku Haki, and has been acknowledged by Sengoku, these three feats alone really do put her in a better league than some of the other shichibukai. After all she has an island of extremely strong warriors, has two sisters of large strength and has effected some really powerful people, Vice Admiral Momonga only managed to avoid her powers by stabbing himself, and the head of Impel Down Magellan was completely besotted with her. Remember we have seen what imbued weapons can do, now imagine what she can do with her snake, perhaps she wields it as a weapon? If this is the case then I think she has more in terms of physical strength then people my think. And just one more thing, her island is in the middle of nowhere, how could she expect to gain a huge bounty if she cant travel too far, Its not wise to judge her on the 80 million as she hasn't had the opportunity to increase it like most pirates have, but despite he small bounty and opportunities she has made a big enough impact to be noticed by the government and be held famous around the world. This makes me think she will be stronger then anyone would suspect.
I would rate Blackbeard and Mihawk at the top, mainly because they don't rely on much more than the skill and strength they hold. I don't know much about Jimbei, Boa Hancock or Donquixote Doflamingo, so I'm going to wait till I can make a better choice of strength order with them. I have great expectations of them all though.