According to Jiraiya, Minato was one of the most gifted shinobi who ever lived. Anko Mitarashi, in discussion with the Third Hokage about Orochimaru's immense strength and his threat to the village, stated that she wished the Fourth had been alive to face him. During the Third Shinobi World War, flee-on-sight orders were given to enemy shinobi should they ever see him appear on the battlefield. Minato was also capable of countering the Nine-Tails' attacks to the point that Tobi needed to step in to face him directly. After their battle, Tobi was forced to retreat due to being severely injured by Minato. He even complimented Minato and stated he was worthy of his title as Fourth Hokage. His abilities were so far advanced that even the Fourth Raikage thought that no one would be able to surpass him.
And Kishimoto has said the he IS the strongest ninja even though he's dead, and has expressed feeling about making a spin-off sereis about his life, and that I'm sure everyone here would see it and would love it