We believe in the Hype U_U
^^^ This basically sums it up.
Also, Kishimoto clearly stated in the manga (whether he regrets it or not) that Hiruzen when he was in his prime was the strongest of all the hokage.
Also, Hiruzen was said to have mastered all jutsu that exist within the leaf (excluding KKG's and their tech), which means this man had a counter for almost every justu you could possible use on him. And if its true that he mastered all justu in the leaf that also means that he is able to use every element.
Also, Hiruzen MASTERED EVERY form of shinobi combat. Which means he is a Master of Taijutsu, Genjutsu, Ninjutsu, Finjutsu, Bukijutsu, and any other form of combat. Which means no one can out class him in any skill because he has mastered everything. Even Might Guy would lose to him in a taijutsu match basically.
Now imagine how powerful this man was with all of this in his prime, when we was young and said to have had Immense (monster like) chakra reserves, and chakra that is so strong that it made the atmosphere dense and destroyed the surroundings (much like hashirama) and can use all jutsu in the leaf, can use all the chakra element, has a large arsenal of ninjutsu and genjutsu. And is such a genius that he is known also as THE PROFFESSOR!! He is the only genius in the manga that actually has a nickname to back it up.
You basically saw yourself how easily he figured out obito's tech by throwing shuriken at it. The man is unrivialed in terms of genius.
What Hiruzen's Prime Feats/Hype basically suggest is that he was unbeatable. That why people put him on such a high standard. And these are all words from the manga itself, so according to the Manga Hiruzen in his Prime is basically the strongest of all shinobi (excluding So6p).
I also forgot about EMNA, but the staff is the ultimate weapon, and kishimoto himself called it the Strongest Summon in the official Fanbook.
