Anyone who says Itachi would win has downs. Get past your fanboy tendencies and realize that someone can beat your "great character." Trust me, I hate Nagato like many Itachi Fanboys do, but there is no way Itachi can handle the 6 paths. Even with Susanoo. They would bust his limits. A half-assed Chibaku Tensei is enough to take him down. He couldn't take it down on his own, and if there was a full powered Nagato, he'd be screwed. But, since everyone is going to complain, let's do a character breakdown.
Itachi's Best Powers
Susanoo
Tsukuyomi
Amaterasu
Pain's best powers
6 Paths of Pain
Shinra Tensei
Chibaku Tensei
Bansho Tenin
Itachi using Amaterasu isn't good enough because another path could get him whilst he's hitting a different path. Pain doesn't feel physical pain so Amaterasu wouldn't do too much to distract him.
Susanoo could most likely be defeated via Chibaku Tensei and or the paths waring out Itachi.
Tsukuyomi is a possibility, but it is possible that Rinnegan is immune to occular genjutsu.
Pain takes this one.