if itachi wants to kill someone, only Sasuke or Kakashi can stop him. amaterasu is unblockable and affects anything within his line of sight. no matter how fast Jiraiya is, all Itachi has to do is look at him the wrong way, and obviously forget about eye contact. also, Jiraiya has said he sucks at Genjutsu which is Itachi's specialty.
but, Jiraiya is definitively a stronger and more well-rounded ninja. Itachi's idea of "stealthy infiltration" of Konohas was to wear giant conical hats, not conceal their akatsuki robes, and obviously eavesdrop. Jiraiya disguised himself perfectly and turned a frog into a fake coffee shop and got info without resorting to hypnosis, not to mention his invisibility technique that, while never shown, was alluded to in a flashback and implied to be how he gathered most of his information on Akatsuki. Jiraiya also has superior defensive capabilities, as well as Taijutsu that's miles ahead of anything Itachi has even shown.
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