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Notice how Hiruzen is the only Kage that doesn't have a unique KKG or technique that would put him above others? A testament to how powerful he is, that he would be named the strongest Kage of his time when he reigned during the time of the 3rd Raikage, 2nd and 3rd Tsuchikage Oonoki and the strongest Kazekage as of right now which is the 3rd Kazekage and the 3rd Mizukage who was also the Jinchuriki of the 3 Tails. Not to mention he was named the God of Shinobi and was known by people as the strongest Hokage to ever live. Further implying how he is stronger than those before him and those after him, the only person I can say is stronger or equal to him is Hashirama, the rest are inferior to him in every aspect other than speed of course hence Minato and Tobirama and their FTG, no KKG, no special techniques, Hiruzen is truly the greatest.