It would have to be the Tsuraunni, Although great kesh is a major force on midkemia they'd struggle to hold back the Tsuraunni as I don't think they would have enough resourses.
Kesh doesn't have a magical force a such, only the odd magician poking about, so they'd struggle against a couple of Great Ones.
The only reason the kingdom held off the Tsuranni was due to them fighting on three fronts, against the kingdom troops, the dwarfs and the elves to some extent. Plus they were limited to the amount of warriors permitted to enter the war by the ruling party. If the full force of Kelewans armies and magicans were up against Kesh, then I think Kesh wouldn't stand a chance.