In my opinion(pure elemental user):
Fire = Madara. Since the Uchiha clan is naturally adapt in fire style and he was clearly the most powerful of any Uchiha. Plus he had access to the EMS kenkai genkai amaterasu, which means he could simply spam the tech with out any consequences.
Water = Kisame - There are few characters who can even survive meeting Kisame and none that can use water style ninjutsu better than he could, he had so much chakra that he could basically summon an ocean and control water way better than anyone else.... Once he's got you 1 on 1 in water, you're dead. Not to mention his Water ninjitsu can absorb massive amounts of chakara.
Earth = Kitsuchi is the most powerful earth user i have seen. This is a tough one because we have not seen that many pure earth style users. But he is to take on hundreds of white zetsu by himself. Was able to make to the demonic statue into a sandwich as well as make a deep trench to in snare the ten tailed beast and he clearly has a huge chakara store.
Wind = Naruto, the rasengshurikan is the by far the strongest wind release seen and the fact that he could create an army of Kage level strenght shadow clones and and spam such a powerful tech proves it. Plus with his new speed in tailed beast mode, Sage Mode, and he's the fastest in the world of Nartuo as well.
Lightning = Third Raikage with out any doubt. His defense, finger jutsu, speed, and chakara levels clearly place this guy very far outside the realm of any other shonobi. Lol he was so strong that only he could defeat himself.