Reasonable is one thing I am, and that is something you are not. If you force your opinion on others just as eagerly, then you are no different.....But I'll give you a reason to believe the way I do. I am not a fanboy of either of these, so I have perspective, and no bias. Both are incredible characters in the Narutoverse.
Six Paths of Pain vs Itachi
This is actually the preferred mode of fighting Itachi I do believe. 6 v 1 is something that would give Itachi mad trouble. Fully awakened non-edo Nagato wouldn't be as effective I don't think.
That said....Itachi's taijutsu means nothing against 6 enemies. His genjutsu means a whole lot of nothing since it's ocular, Nagato simply commands the Paths not affected to stick a rod up his ass.
Now, as to the Susano'o/Yata Mirror/Totsuka blade argument....You saw Itachi's burns after getting hit with Kirin, correct? He manifested the Susano'o atleast to some degree to block Kirin, and the Yata Mirror certainly helped. It is by NO MEANS an impenetrable defense. If Kirin can bite through the Mirror and atleast a partly formed Susano'o, so can a Village-Busting Shinra Tensei. Also, we must take into account how the Yata Mirror works. It changes it's own nature to offset the incoming attack, meaning it fights fire vs fire, lightning vs lightning, etc.
With this being said; Can Itachi win, yes....he can. But would he? I doubt it. It's quite simply due to the things that he lacks rather than what he has. He is a master at most every form of combat. Has quick-kill capability....So does Nagato. Furthermore.....We have no way to confirm or deny whether Shinra Tensei in any capacity can be blocked by the Yata Mirror, seeing as how it isn't an element. But, that being said.....Everything has its limit. Yata Mirror reached it's limit with Kirin. That Kirin pushed it that far means Shinra Tensei or Chibaku Tensei would do the same. Bee's TBB was what destroyed Chibaku Tensei, not the RasenShuriken or Yasaka's Magatama.
Itachi alone has chances at victory, but everything must fall into place. As we all know, things rarely do. I'm not discouraging debate by any means....This is just my viewpoint.
Six Paths of Pain would certainly win it from where I'm standing, Fully Awakened Nagato would win as well, simply due to his near Godlike abilities, far transcending the MS. Again, this is just my opinion.
