Pain actually being Pain could beat Itachi imo. The two fought in the manga but it was basically Itachi in his prime with infinite zombie chakra/stamina against Nagato as a cripple with no paths and Itachi won.
Give them human form and actually give Nagato the tools that make him good (his zombie paths) and I think Itachi is getting beat. 6 vs 1 is a hell of a fight for anyone.
