I'm assuming Healthy Itachi is just him not puking up blood and such.
Pein wins, all six bodies would overwhelm Itachi. Itachi's genjutsu shouldn't really work on Pein for a couple of reasons.
1) Pein would have knowledge of Itachi and him being a genjutsu master, thus being cautious with him.
2) Pein is theoretically immune to Genjutsu. Each body is being manipulated by chakra that is being sent in by Nagato. Nagato, if he suspects any path is under a genjutsu can alternate or stop the flow/stop sending chakra to the path. Breaking the path out.
Amaterasu can be absorbed by the Preta Path or dealt with via Shinra Tensei. The Asura path would pulverise Itachi in close quarter combat. The Animal Path can summon the dogs, bird, rhino, etc and distract or weaken Itachi that way.
Susanoo is a temporary ability that can either be dealt with by the Preta Path absorbing it, or a Chibaku Tensei.
The shared vision from all of the paths and summons give Pein too much of an edge when fighting. And the abilities each path have working together at different angles. . . Itachi won't make it more than a few minutes.
