Up to speculation but:
Jiraya > Itachi, mostly because Itachi himself said so.
Itachi > Orochimaru, mostly because Oro said so.
Orochimaru > The Third at his end.
Orochimaru > Tsunade (come on she couldn't beat an armless opponent? or even his jounin subordinate?)

So Jiraya is the most powerful so far.

I think Tsunade >> Kisame. Kisame may be Atasuki, but he is not enough to be on par with a sannin.

Even this order leads to some confusion however. If Itachi was so damn strong why didn't he kill the Third? Or does he not care about political power?

Of course Itachi > Orochimaru might just be because well a Snake is more easily charmed/destroyed by Itachi's sharingan?