I am not saying that Itachi would win just because he's my favorite character (but he is, lol), but Itachi has actually NEVER showed his full power! Even when with Sasuke he was blind/sick/and forfeiting! Here's a scenario:
-This is at the end of the battle when they are pushed to their limits-
-Itachi uses Amatarasu!
-Pain tries to dodge!
-Itachi focuses Ametarasu!
-Pain is beginning to be engulfed by the black flames!
-Pain uses his ultimate to obliterate the flames! *that is, if his ultimate jutsu is actually an explosion like his Shinra Nuke no Jutsu*
-Itachi escapes and uses Tsykuyomi while Pain is taking his breath, thinking its over.
-*in-case you forgot, Genjutsu is the way to beat Pain. And the last time i checked, Itachi RAPES in the Genjutsu department*
-Itachi tortures Pain quickly before Pain shatters his Doujutsu with his Rinnengan
-Pain uses a few more techs and knocks back Itachi into submission
-Itachi closes his eyes, stands up, and Susanoo is beginning to form
-Pain, in shock, uses the last of his power to create a Shinra Tensei Nuke but the Susanoo Sheild blocks it.
-Pain tries to use taijutsu but Susanoo pushes him back
-Susanoo Attacks with his sword--Totsuka no Tsurugi (十拳剣, Ten Hands Long Sword), also known as the Sakenagi Longsword (酒刈太刀, Sakenagi no Tachi; Sake Cutter Longsword!!!
-Pain tries to block it with his Shinra Tensei shield!
-Pain epically Fails and, in doing so, the sword thrusts through his body and kills him
-Susanoo slowly deforms and vanishes and Itachi walks away, victorious
EDIT: Yes i realize that the Realms of Pain are a trouble too but Itachi can handle that with his Futon jutsus, Doujutsu's, Hand-to-Hand, some other Ninjutsu, etc.
