An interesting thought occurred to me when reading chapter 698. Law was explaining how each of the Yonkou held certain territories (which we already knew), and how each of them had been growing them even more in recent months/years. This got me thinking about what the power rankings of the 4 Yonkou might be, and whether they have changed since before the time skip.
Note: I'm not talking about them as individuals here, but rather the combined strength of their fleet and allies.
We know that when White Beard was alive he was called the "the closest man to one piece" which could also be considered as suggesting he was the strongest of the Yonkou. The fact that he had a fleet of 1600 men himself in addition to the 20+ allies/vassals ships that showed up at Marineford was likely the key reason for this. After WB's death we don't know what happened to his crew, but I would make a guess that they may have allied with shanks, thus boosting the overall strength of the latter.
If that is indeed the case, then the other Yonkou would have to react accordingly. Big Mam probably incorporated several of WB's old allies, since she also seems to have taken some of his old territory (Fishman Island) as well. BB already has an ungodly strong crew after the addition of the criminals he broke out of Impel down, and I think he was mentioned as recruiting DF users as well. This just leaves Kaido. I can't help to think that his deal with Joker and Ceasar to develop artificial DF's for his crew wasn't so much a crafty ploy to give him a edge over his competition, so much as it was a desperate plan to help him keep up.
For this reason I believe Kaido may actually be the weakest link among the Yonkou in overall crew strength, and thus a good choice on Oda's part to make him the first major enemy of the SH's. If the SH's and Law's crew together only have a 30% chance at victory against the weakest Yonkou, then there is still plenty of room for further character development in the future, which is a good thing IMO.
So what's everyone predictions on the Yonkou rankings? Mine are:
1. Black Beard
3. Big Mam