I've always given this particular matchup to Pain. They're definitely equal opponents but the Pain set-up means that any battle against the six of them will never be over quickly. Naruto, Konoha, the 5 strongest Toad Masters and the Kyuubi were all needed to whittle Pain down- and that still took ages and every ounce of chakra and energy.
Itachi doesn't have that much, unless genjutsu works on shared vision (which i'm pretty sure Jiraiya's Genjutsu disproves) then Pain overpowers and perserveres.
I don't know much about Nagato but I imagine him losing to Itachi just as quickly as the fight happened in Edo form.