Pain and whoever. I picked Naruto though because of his strength/durability/stamina in Sage Mode plus the summons he brings. With Pain though or really Nagato in this case, you get a person who can control life and death. Nagato himself or his Naraka path will just revive Naruto whenever he gets killed. It's pretty much a no-lose contest and pretty much unfair since both of them use multiple bodies to fight in their battles.