Orochimaru in peak form is weaker than Itachi. He said as much himself. Likewise, Itachi said that, in a fight against Jiraiya, the best Itachi could hope for would be both dying, and more than likely Jiraiya would kill him - even if he had reinforcements from other akatsuki groups. Therefore, Jiraiya > Itachi > Orochimaru. Tsunade would probably be even weaker than Oro right now, but that's only cause she's out of practice. Who knows who would be stronger otherwise.

To back me up here: "Yes. If we both fought him, we would both be killed...or in a good case, we might kill him, but we'd die along with him." -Itachi speaking about Jiraiya from the AA trans of 83