Nagato would win of course...
The only way for the third to win would be if he has previous knowledge and some super op jutsus but even then, fighting full powered Pain or Nagato, with Nagato having the intent to kill him, he hardly has a chance...
The third wins, Nagato is super overrated. Naruto took out a ton of pains and Hiruzen is said to be the strongest Hokage. Go ahead and say "Only Iruka said that!" Kishi included it because it was information he felt we needed. You can't act like Iruka controls what he says, its Kishi. And no statement has contradicted that exact claim. So I take Hiruzen.