Without reading all these overly emotional replies I'll weigh in on Itachi vs. Jiraiya (whoever said he'd lose to Kisame or Oro are being silly)
Itachi himself said he didn't want to fight Jiraiya, but I mean he said that about old man Jiraiya so it probably had more to do with he wasnt in the mood to fight a powerful dude when his mission had already failed. Also he was a double agent so why kill one of the strongest good guys left to defend the leaf. You have to consider though that Itachi's greatest asset, his overpowering genjutsu has no effect on Jiraiya (sage mode.)
And we've seen Amaterasu isn't that effective anymore either (higher level ninjas aren't fazed by it.)
So it'd come down to Susanoo and what ever other tricks Itachi pulls out -- legendary weapons and artifacts, random *** eye techs etc -- against Jiraiya's sage mode.
Considering Pein who had the rinnegan agreed if Jiraiya knew his secret he'd lose, I think it's safe to say prime Jiraiya would have beaten prime Itachi. It's not a knock on Itachi, he's anywhere from the 2nd to the 7th strongest character we've seen in the manga.
But he's not unbeatable.