A (great, speed and power but he is easily agtated)
MEI(water mastery and 2 kgs)
GAARA/TSUNADE(dabtable both need to prove them selves more)
OONOKI (he is old now)
You think that Onoki is last? I'm sorry but you might be the only one. I can give you dozens of situations where Onoki easily kills all the rest of the kages. Not at once but one on one.
Onoki might be old but he is the strongest kage.
A can't beat him because A's attacks are purely nintaijutsu and A can't fly. All Onoki needs to do is fly up and use dust release on the battlefield and A loses. Onoki can also make A really light so his attacks aren't powerful, or he can make him super heavy so that he can't move.
Gaara has literally no chance against Onoki. Gaara's only means of attack and defense are his sand, his body is super weak. Do you think his sand can protect him from dust release? Not a chance. Not only that Onoki can turn the sand extremely heavy. He has already changed the weight of Gaara's sand once so we know it can be done. If it were heavier it would move much more slowly and Gaara would waste more chakra moving it.
Mei - two KG's vs. a kekkei tota. Kekkei tota wins this, especially since Onoki can fly. Her only chance is to use hidden mist and hide, at which point Onoki would dust release everything.
Tsunade- has no chance. None. She can't fly, she isn't fast, has no KG, has one huge and slow summons and is strong. She can heal herself, but not if she gets hit with dust release. Dust can't heal itself.