Jiraiya knows of Itachi's genjutsu (as he told Naruto about it), so he would be prepared. He would quickly enter Hermit Mode. Itachi would fight him and find out the strenght advantage is clearly in Jiraiya's favour. Then, he would summon Susanoo. Jiraiya would as an answer summon Gamabunta. Of course, Gamabunta would die. Itachi is protected from all sides, so what can Jiraiya do? Attack from below and kill the unprepared Itachi with Swamp of Hell.
Truely an advanced match, as both have advanced powers. It could go either way.
I don't view Naruto as manga. I see it as a comic which is not by an american author, but japanese author. If the mods disagree, then they are racists, pure and simple.
I don't know if summoning is a legal tactic in a straight fight due to one on one rule. If Jiraiya cannot get help from the elder frogs he would not be able to enter in hermit mode.
Summoning jutsu is just an attack like any other jutsu. The attack is living, but it's still just a part of the ninja fights. Some fights were fought solely on summons, like Third Hokage vs. Orochimaru.
Jiraya cannot use genjutsu by himself, he would probably go down with black fire, Sasuke survived that one only by using one of Orochimaru's best techniques.
But Jiraiya is prepared for Itachi's genjutsu. So it wouldn't really matter. It would all come down to openings in their final tactics, where I believe Swamp of Hell would counter Susanoo.
i think itachi would win..if he use tsukuyomi on jiraiya..it will all be over...from what i know..only those with the sharingan can counter it..even if it didn't work..he still have amaterasu and susano...even if jiraiya is stronger...with susano...physical attack won't work..
The point is, Jiraiya has the nastiest tech in the long run, his secret song jutsu would pass through Susanoo and screw Itachi, but it won't come to that; Itachi doesn't need to go defensive if he can just use the tsukuyomi and end the battle.