I don't think itachi will ever figure out what jiraiya figured out during their fight.Itachi's Susanoo IS the perfect defence due to his items (Yata Mirror) and a brilliant offence due to the Sword of Tsokuka. As Itachi's chakra reserves are that big, I would give it to Nagato. Also, as Itachi doesnt have prior knowledge, it would take him some time to figure out the real Nagato is none of the Peins.
I give it to Itachi (very,very close) as hes shown to be one of the most intelligent characters, highly perceptive and very, very skillful. His fight against Sasuke was one of the best so far, and he didnt even exert himself much as he didnt want to hurt the brother hes been protecting for the last 10 years or so. The guy was near death (illness), half blind and still matched Sasuke without even trying particularly hard. Orochimaru himself stated Itachi was stronger than him and Itachi absolutely pwned Oro 8/10 years ago. Based on achievements, what we have seen, and guessing individuals' true power, I would say Itachi has the upper hand.
Although, Pein is shown to have the Rinnegan and massive chakra reserves (able to control 6 bodies etc..). He is able to use some of the tech the Sage of Six Paths Himself used, which is damn impressive in its own right. Also, Im not sure if Susanoo can defend against Pein's giant meteor move.
Very close, but I give it to Nagato.
coz itachi never met this ninjas before... jiraiya only realized that nagato isn't there because he remembered the faces of this shinobi in his past.
this will allow NAGATO TO WIN EASILY.
come at me fanboys.