This may not be an accurate list of the most powerful Naruto characters as the names here may or may not be able to defeat everyone in Naruto universe based on different situations. The characters, however, have great skills and impact on the audience. You can watch high quality anime online and check out all these characters throughout the seasons of Naruto.
The saint and the edgiest character in Naruto universe, Itachi lives on in the hearts of people even after his death. He has a great fan base who believes Itachi could have ended the great ninja war if he was alive, but we can never be sure as the writers thought of ending that arc way before. The death of Itachi probably makes him the most loved character and not to spoil the things for new watchers, but Itachi was always a good brother.
Another great Uchiha of the leaf, Madara is probably the strongest Uchiha in his ultimate sage of six path stage. However, Might Guy gave a tough fight after opening his Eight Gates bringing him down to the ground. But we cannot base our judgements on just one small battle when Uchiha Madara was solo fighting the war against five nations while holding back on his powers for the ten tails beast.
Jiraiya was the mentor for some of the greatest shinobis like Naruto, Minato, and Nagato. He was one of the strongest characters in Naruto universe. Jiraya faces an unfateful death from Pain, due to his past. But he mastered the sage mode, and only a few could match his level of chakra, like his own sensei, the Third Hokage.
The First Hokage, Hashirama Senju is considered the strongest shinobi and not to forget that he fought head-to-head with Madara, twice, and won both the time. He possed the rare wood Jutsu which was his final blow to defeat Madara.
One of the founding members of Akatsuki, Nagato was the disciple of Jiraya who went through the trauma of losing his best friend Yahiko and hence decided to join Tobi for his secret motives. Nagato possessed the Rinnegan the most power eyes, earlier known to be possessed by the sage of six paths himself. Nagato referred to himself as “Pain” when he attacked the Hidden Leaf Village. However, he was later talked down by Naruto, which brought an end to his pain and suffering.
Uzumaki Naruto is the most loved character of the series for the obvious reasons. Throughout his journey, he gained more and more chakra to become equivalent to the sage of six paths. Not only he became strong physically, but he also won the hearts of people to become the best leader of all time. Not to mention his biggest victory, getting his friend back home.
It would be unfair to not mention the shinobis below if we are talking about the greatest characters in Naruto series.