Honestly unless Hashirama has some godlike abilities that we have not heard about then there is no way he could possibly win this. We don't know that much about him but we have a basic understanding of his jutsus that we was famous for.
From what we have seen there is literally nothing he could touch Nagato with.
I understand why some people say Hashirama but all of that is unfounded and just hype.
Nagato however has all of the abilities of a regular ninja, probable the most chakra out of any character that isn't a jinchurriki, and on top of that he has the rinnegan.
What can Hashirama do to the jutsu that steals chakra? Hashirama does not have a bijuu's amount of chakra to give away like Bee, nor does he have natural energy to turn Nagato (or his paths) to stone. So that is one way that nothing can touch him, because in case you forgot he can literally absorb any and all ninjutsu. So nothing Hashirama could attack him with that used chakra would work. Sayonara mokuton, hasta luego, see you later.
In addition to that he also has the ability to repel attacks and people, or bring them in using gravity. If he used that on him and then just absorbed his chakra then Hashirama would have no chance to do anything, not like he ever did. Oh I forgot he also has the giant chibaku tensei, where he makes a miniature version of the moon with intense gravity that crushes the opponent, Hashirama has no way of destroying that on his own and would die by it.
Then Nagato can make his body armored with his Asura paths abilities, and he gains weird mechanical appendage things that can attack.
Also he has a huge amount of summons, one of which multiplies every time you cut it. And another that can turn invisible. Hashirama has no summons that we know of so he loses on this front as well.
And in addition to his other rinnegan abilities he can also steal one's soul thereby instantly killing you. And he can regenerate his paths or heal himself with the whatever path.
And then if that wasn't enough, if he didn't want to use the rinnegan he has an affinity for every element and can use any jutsu he wants to (provided he knows it).
Hashirama is outmatched and there is honestly no case for someone to argue that he could win. I personally am not really a fan of Nagato because he killed Jiraiya and didn't bring him back, but I will be the first to admit that he is one of the strongest in the manga and taken on one v. one he is practically unbeatable unless it is by an already kage level ninja and they get extremely lucky in addition to Nagato making alot of mistakes. However that is extremely unlikely so yeah Hashirama does not stand a chance.
Nagato beat all of Konoha, Naruto in sage mode, fought Naruto in the six and eight tailed modes, and was still alive to face him afterwards and had enough chakra to bring back everyone he had killed.
So unless you think Hashirama could beat every single person in Konoha, Naruto in Sage mode, and Naruto in two different versions of his tailed beast mode then he loses. If you do think that then I can't help you as you obviously don't understand the difference in power between the two.
I mean it kind of annoys me how many votes Hashirama has because he is only hype, he got beat by Hiruzen at 68 alongside his brother. Now that may have been a weaker version but still that is pathetic by any standards.
Nagato on the other hand fought Naruto in his kyuubi chakra cloak, Bee (a perfect jinchurriki), and Itachi; and was soundly beating Naruto and Bee and until Itachi sealed him.
I just want to know how anyone thinks Hashirama could win. All I have seen is people saying "oh I am sure he could do it because I am a fanboy and he beat Madara with EMS which we know nothing about, and he had to have the fox sealed into his wife so he could fight Madara on equal footing"