Its obvious hes the strongest. He is like the Sage of the six paths at this point...
Making Moons fall from the sky back to back like it was nothing... you can see the difference in power between him and Nagato.
As strong as Naruto could ever get... there is just no way one single being will be as strong as this... it's pretty crystal clear that Madara will need to be taken down by both Sasuke and Naruto.
Hashirama is hella strong, but current Madara has Hashirama's arsenal, + EMS and those two together should already make him stronger than Hashirama. Add the Rinnegan and you have a much larger gap.