Six Paths of Pain mid diff (if no knowledge), as not one of the Kages has shown much intelligence; A is just retarded but powerful, Tsunade is just there, Ooniki is a good Kage, Gaara is reasonbale, Mei we dont know enough of.
Even if some of the Paths are beaten (which will obviously happen) Naraka Path will just revive them. Without prior knowledge, Six Paths of Pain are virtually undefeatable (only current Tobi, Edo Madara can almost definitely beat him. Obviously Kabuto + Orochimaru + Tobirama due to Edo Tensei). Even if one finally figures out Pain's secret, it'll most likely be too late (e.g. Jiraiya).
If there is prior knowledge, still Six Paths of Pain but very high diff. Firstly, Kages dont have anything to stop shared field of vision, summons alone will keep 2 of them busy, and other Paths are too good for the remaining Kages as well. I rate all Kages (apart from Ooniki) below Jiraiya, and he had an incredibly difficult time just dealing with the summons.