When Naruto and Sasuke fight each other in the valley of the end, it was mentioned that the Valley of the End was called by that name because of the best friends who fight with each other before the Konoha Village was founded. It was again mentioned that these best friends were the First Hokage and his best friend.

I thought the opponent of the First Hokage here is the Second Hokage. However, it was again revealed that the Second Hokage was the younger brother of the First Hokage.

Well, thanks to the Naruto Manga chapter 370, for the fact about it was already cleared. According to the chapter 370 through the story of Jiraiya, the two bestfriends who are fighting with each other were the First Hokage who was the founder of the Konoha and Uchiha Madara who was the founder of the Uchiha clan.