Kishi said Madara was the strongest and unbeatable = fact. He had to use plot to rationalize his defeat, that being Black Zetsu. People forget one thing... it was KAGUYA who created Black Zetsu, one could say it is an extension of her and it even rested in her sleeves, so I'd consider them one and Black Zetsu guess what, pwned Madara. So Kaguya > Madara in terms of pure power. In terms of battle experience and skill, it's Madara, no one is better in that department, Madara was essentially a war-machine, he spent his youth on battlefield.
Kaguya is the very strongest, only Six Paths: Chibaku Tensei can get rid of her. It took both Hagoromo and Hamura to beat her and ONLY thanks to said technique. Hagoromo somehow later on got both parts of it. But would Hagoromo be able to fight and seal Kaguya? Answer: no. He was a priest and he needed help from Hamura when they both were young, I pretty much doubt grandpa Hagoromo if alive would be able to counter her, being old and all.
@bold madara already countered her weakness SPCT by substituting his doppelganger its just half of his power (1eyed jin madara)