@Rotten:
I doubt Joker will make it in time. Otherwise he'll just get his ass kicked. There's no chance he can take them all at once. I even doubt he can take luffy.
You gotta remember that out of the original seven Warlords, he was always hyped as the most powerful. For the longest time he had the highest known bounty in One-Piece.
More importantly, if I am remembering correctly, he was the only person to come out of the war without so much as a smudge on his face. Doflamingo is insanely powerful simply because he can control people without even breaking a sweat. He doesn't even have to fight. Now just imagine what would happen if he got control of Luffy, Law, and Smoker.
I honestly believe that they will urn into each other. Dofla will be winning, but something will happen to allow them to get away. This will allow Luffy to see his power and figure out how to beat it the next time they fight.
@MiyamotoMusashi:
Not really, you have to understand that the Yonkou are governmental themselves. They completely rule over the NW, the WG and thus the Marines have, at least until now, nothing to do there, so as a Yonkou he also could do anything he wants. So, a Yonkou post would not be disadvantageous for him, for someone addicted to power like him, it would be a very good position before his ultimate goal, namely being PK. He just does not have the power to take a Yonkou spot, that´s all. Just think about this: Vergo, his most trusted and loyal subordinate, was finished off by Law rather easily. 2 other members of his crew are fighting Franky who at least can hold them back, if not even fight equally. Monet was humiliated by Zoro. Now compare those guys with characters like Marco, Jozu, Ace or even Vista who could hold off Mihawk and you get the gist why he can not be a Yonkou. Whether he himself is as strong as one is also doubtful.
A Warlord is still more advantageous, because he has the world government on his side. And, as Joker, he has at least one Yonkou on his side. And who knows hom many countless people are working for him. His ability to run an operation of this magnitude shows his intelligence, and his capability of making it through the war unscathed while laughing and playing around, shows his skill and confidence.
Doflamingo is not a man to be underestimated. I have no doubt that he is the most powerful man in the One-Piece world, even if that power is not pure strength.