Pro-Itachi Argument
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Sharingan genjutsu seems to be image-based. If you see the image, the genjutsu takes you:
which is why projection works. Nagato is essentially having Pain project images for him in a security-camera panel, meaning that Nagato is potentially vulnerable to Sharingan genjutsu prokejection, like Tsukuyomi. The Frog Song is a different type of genjutsu, but it may have also affected Nagato:
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Nagato said "you got me" and was chatting inside the genjutsu world while his entire body and Pain was paralyzed so that they couldn't move an inch. Hence Nagato was in the genjutsu speaking, because Animal Realm lacks a mind of its own. This is supported by his late attack with Asura:
after Jiriaya
summoned those swords to stick in the Pain bodies. Coincidentally, the databook says the genjutsu isn't typically lifted until the swords were already in, which means that was when Nagato was set free, and allowed him to attack through Asura.
Inside the genjutsu world, the target is surrounded North, East, South, and West, by giant toads, imprisoning them in a barrier. The four unmoving toads won't lift the complete binding until the user gives the order. All that is left to do is to strike the finishing blow in the real world. →Mt. Myouboku's secret stone swords³ are driven into the targets' hearts like a wedge. The targets are finally released into the realm of the dead.
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Itachi dragged and sustained his Amaterasu,
lighting an entire forest on fire with one Amaterasu. The benefits of sustaining and dragging his Amaterasu against multiple opponents instead of trees, such as the Six Paths, is obvious. If Nagato is avoiding his eyes, then he probably can't react like
A did. It's highly unlikely Nagato can match his A's shrouded reflexes, and Itachi's Amaterasu burns
hotter than Sasuke's thus far, and was activated while running at full speed towards a wall five feet away (very quickly.) It can cause some issues, but more importantly, it can be used
in conjunction with Susano'o defense if necessary.
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It's a common mistake to assume Susano'o is a primarily defensive jutsu. It's not. The databook first and foremost hypes the
perfect flawlessness in offense that presumably surpasses his other two techniques. It caught Orochimaru in between words, right in front of him, without him even noticing, when other speed demons get
whole sentences off. Kakashi and Danzō had to use seal-less jutsu to deflect Susano'o attacks because they were so unavoidably fast, and Sasuke only recently attained
Itachi's level of Susano'o,
which the author decided was a power-up to save for later. The Yata Mirror (and probably Totsuka) can extend itself as required, as it is formless, and change nature and form to counter any jutsu. The databook indicates there is a possibility that
Shinra Tensei would be rejected, as would Preta's ninjutsu be countered by the legendary item.
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Lastly and most importantly, Itachi is possibly the greatest genjutsu user to ever exist, eclipsing even Madara by reputation. Ao and Shikaku implied he can take control of people at large ranges. Considering that Nagato is looking through six cameras, if four of those cameras were infected with a virus producing false imagery, then
Pain might begin attacking itself. That isn't to say that Pain is weak against genjutsu, or any genjutsu user...it's just that Itachi is the very best.