It annoys me when people say so and so is beyond Kage level. Many Kage are above Kage level, they just hold the title.
That being said, Itachi and Nagato are one of the most unstoppable two man teams in the Naruto universe, the only teams that could even think to contend with them is Hashirama/Tobirama and Hashirama/Minato/Hiruzen/Madara/Tobi(all interchangeable). Five kages, in my opinion, would be too much for any two man team.
Onoki is really the main thing. If Onoki is at "full power" as the OP says, that means he is younger and in his prime. Seeing how well he has performed at his age, I think an Onoki in his prime would be pretty beast. Tsunade can heal everyone, while using taijutsu on the offensive. A is super fast(too fast for genjutsu he stated, though I do not underestimate Itachi's prowess) and strong. Gaara has the ultimate defense. Mei has two OP elemental releases. I have to give the slight victory to the Kage, though I believe it would be a struggle.