So now it seems that we are finally going to see a Saga being based on Do Flamingo. And it will undoubtedly be one of the most interesting as it seems that not only is he damn strong, but also has tons of strong underlings as the guy like Vergo, who seems to be around average Vice Admiral level and the one who is casually able to take down a guy like Law who also managed to take down Smoker, is his underling and we may see even more guys of at least Zoro's and Sanji's level as Flamingo's underlings, thus we might assume that the same Do Flamingo should be at least around strong Vice Admirals or even very close to Admirals in terms of power.
Also he has quite a big support in upper tiers of WG as it seems from his talk with the guy after the War in Marijoias.
So we will finally see Luffy progress far beyond his current level after this Saga.
To be honest, it's a tossup between Blackbeard and Jimbei for me. Blackbeard has Whitebeard's power, which could apparently destroy the world. With his yami yami no mi, he can render most devil fruit users useless and be able to suck in anything.
However, Jimbei would probably be a good match for BB, at least before guro guro no mi (?). His power comes from fishman karate, which is pretty effin' powerful even out of water. Jimbei never ate a devil fruit, which gives him an advantage, as well as being able to attack from afar. Though, I think BB can take that attack with his dark hole, but close combat Jimbei wouldn't be in as much trouble.
1) BlackBeard. He's one of the main antagonists, and he has a really fricken cheap DF. He may have even more than just the Darkness and Earthquake fruits. Who knows? Maybe he can have infinite fruits. But since he's quit, I don't think he counts.
2) Doflamingo. I think Doflamingo is the strongest because of many reasons. He's "Joker," and seems to be a government agent as well. He controls the underworld, while working for the government. Also, his bounty was the highest, and he's mentioned a lot as a very powerful Shichibukai.