Kakashi would be the best option. Naruto and Sasuke will fight over the title of the 7th Hokage, not the 6th. Kakashi is way more mature than them.
Naruto lacks practical logic when it comes to politics... He would be kind to everyone, like Hashirama, which would produce chaos and anarchy, and everyone would do whatever they want.
Sasuke on the other hand, is too cold to be loved by the nation, and would make a rule similar to that of Tobirama, were he would make Konoha a strict militarist nation, with cold executions of whoever is against his laws, and would provoke an other war with his cold logic.
I think the best option would be if they shared the 7th Hokage Title, as one completes the weakness of the other.