how would we know? we only have the current ones ( and most don't have too much detail), only 2 dead ones from villages outside of the leaf and the hokages. we are more likely to see the hokages, so we are more likely to pick them (i.e. minato) and if we talk about the current ones, there are only around 3 we know about. we can't make an educated guess because we don't have even half of the information.
I love gaara to death, but he's really not THAT strong compared to the otherss.
We haven't seen all of the tsuchikage's stuff, but i think he'll be high up there.
Otherwise, prob raikage, despite the missing arm
Of current Kage, physically speaking I don't think anyone is close to The Raikage A. Now Mei Terumi the Mizukage has some powerful Kekkei Genkai in Lava and Boil. Her jutsu is fierce. Onoki may have been stronger than the others at his best, but not now.