I know this might seem insane to you, but I think Sasuke honestly could have defeated Pain.
I was PMing Twisted-Alien and he asked me "How long do you think Itachi or Sasuke would last agaisnt Pain?"
Then I started thinking unbiasly, instead of my usual "PAIN OWNS ALL" attitude.
Hear me out before you say "Oh my god, you noob Pain is the strongest person we have seen in the show."
I agree with that, but that doesn't mean he would win in a fight.
Here are some of the reason I honestly believe that Sasuke or Itachi could beat Pain.
Note I am not including God Realm in any of these.
During Pain's fight with Jiraiya, Jiraiya found out a "weakness" that Pain had. Gen-Jutsu. In fact Jiraiya killed three of Pain's bodies using Gen-jutsu. It is a well known fact that Uchiha's can cast powerful Gen-Jutsu simply looking at his/her opponents. So, unlike Jiraiya, Sasuke or Itachi will not have to run around and wait. This would be interesting to see because since all the Pain's share the same vision, would Sasuke or Itachi only have to use Tsukuyomi on only one of the bodies in order for all the Pain's to feel the effects. Would it go directly to Nageto. It's just something to think about.
Secondly, I noticed a flaw Kakashi's ability to see Charka through his Sharingan. Since Pain uses Charka receivers, to control the bodies, a TRUE sharingan user should be able to see where the chakra originated from. Since the Sharingan can see chakra, Sasuke or Itachi should have no problem finding Nageto. And since Sharingan slows the movements of the opponent, there is no way they would get hit with Chakra Disruption Blades like Naruto and Jiraiya did.
Amaterasu would be a huge problem for 4/6 of Pain's Bodies. With the exception of Fat Pain (can't remember his Realm) and God Realm I don't see the other Pain's avoiding this. None of the Pain's have shown the speed needed in order to avoid getting hit. Since Fat Pain absorbs all Nin-jutsu he would survive and God Realm is... Well he's God...
Last, but certainly not least, Susanoo would be the death of Pain. Itachi's version would own pain, since it has that sword and mirror shield. Susanoo has been shown to be both an Offensive and defensive beast. It was able to protect him from flames that burn like the sun and powerful enough to distract multiple high level ninjas, even in a none complete form.