Honestly IDK.
Mei has the worst moral when fighting, but she easily has powerful moves.
Raikage has extreme speed an power, but is stuck with linear modes of attack.
Tsunade is capable of healing herself, but doesn't have the speed of the other Kage.
Gaara has a near absolute defense and a very capable offense, but lacks expirience.
Oonoiki has flight and Jinton, but does suffer occasional back spasms.
I would say against the other Kage the Raikage has the weakest fighting style. Tsunade can tank his attacks. Gaara and Oonoiki can fly, thus forcing him to become vulnerable in the air....negating his speed advantage. Mei can use her boil release to create an area that he would melt in if he enters....she can also combine that with lava.
The overall weakest against a majority of opponents.....probably Tsunade as she doesn't have the speed of the other Kage, and she is also limited to close range where the other Kage have range, and Raikage has the speed to make up for his fighting style.