I was never a believer of Prime Hiruzen... But I got to thinking what if it all was true. After I did so, I found that it's actually not that hard to imagine....
As Hiruzen got old he lost a step. So let's say in his prime he was as quick on his feet as say, KCM Naruto.
With access to the 5 elemental natures and all reasonable jutsu in Konoha as well as his Enma staff and KCM Naruto type speed how bulky is this guy?
Why do you think Hiruzen immediately knew Jubito was combining all the nature elements to use the Truth Seeking Balls? He did that himself in his prime, thats how.
Why do you think Hanzo spared the Sannin? He feared he would face Prime Hiruzens wrath had he killed them.
Why do you think Orochimaru felt he needed all 3 past Hokage plus himself to defeat Hiruzen? Because he was that strong.
Why do you think he was stronger than all the Gokage in the war arc of part 2 when he was in part 1? Because hes that strong.
Hiruzens prime is something so insane that it will drive you insane thinking about. Please pay your respects to the God of Shinobi.
Why do you think Hiruzen immediately knew Jubito was combining all the nature elements to use the Truth Seeking Balls? He did that himself in his prime, thats how.
Why do you think Hanzo spared the Sannin? He feared he would face Prime Hiruzens wrath had he killed them.
Why do you think Orochimaru felt he needed all 3 past Hokage plus himself to defeat Hiruzen? Because he was that strong.
Why do you think he was stronger than all the Gokage in the war arc of part 2 when he was in part 1? Because hes that strong.
Hiruzens prime is something so insane that it will drive you insane thinking about. Please pay your respects to the God of Shinobi.
Forgive me for I was blind. Praise be to Lord Hiruzen and may he forgive me for my ignorance. I ask that he only keep in mind that I was as a child born without sight, just as I would never know the value of color I could not know the immensity of his greatness
Why do you think Hiruzen immediately knew Jubito was combining all the nature elements to use the Truth Seeking Balls? He did that himself in his prime, thats how.
Why do you think Hanzo spared the Sannin? He feared he would face Prime Hiruzens wrath had he killed them.
Why do you think Orochimaru felt he needed all 3 past Hokage plus himself to defeat Hiruzen? Because he was that strong.
Why do you think he was stronger than all the Gokage in the war arc of part 2 when he was in part 1? Because hes that strong.
Hiruzens prime is something so insane that it will drive you insane thinking about. Please pay your respects to the God of Shinobi.
Translation: Hiruzen earned the title of professor for being a researcher. He was trained by both Hashirama and Tobirama, as the man that would succeed them. At a young age he showed greater talent than Tobirama. All five nature alterations, Hidden, Genjutsu, all ninjutsu existing in Konoha which explains his reputation as the professor. Hence the essence of Hiruzen is his talent with ninjutsu. Everyone in the village is his family member
He knows all techniques in Konoha. Three databooks cant be wrong
Viz Scan Orochimaru - "Knew and could perform all existing Konoha Jutsu" Hiruzen knows all Hidden Leaf Jutsu which equals =
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Hidden Leaf Tech
Neutalizer technique, , Bringer of Darkness, Great Clone Explosion and all 8 Gates at will.
Water Clone Technique
Water Release: Water Colliding Wave (Anime only)
Water Release: Water Dragon Bullet Technique
Water Release: Water Encampment Wall
Water Release: Water Trumpet
Demonic Illusion: Hell Viewing Technique
Earth Release: Double Suicide Decapitation Technique
Earth Release: Earth-Style Wall
Earth Release: Hiding Like a Mole Technique
Earth Release: Multiple Earth-Style Wall (Anime only)
Eight Gates
Evil Sealing Method
Eye Mind Reading
Front Lotus
Lion Combo
Multiple Shadow Clone Technique
One Thousand Years of Death
Shadow Clone Technique
Shadow Imitation Technique
Water Needle
Shadow of the Dancing Leaf
Silent Killing
Summoning: Earth Release: Tracking Fang Technique
Wind Release: Great Breakthrough
Hiruzen (sarutobi clan)
Fire Release: Ash Pile Burning
Fire Release: Great Flame Technique
Dead Demon Consuming Seal
Earth Release Shadow Clone
Earth Release: Double Suicide Decapitation Technique
Earth Release: Earth Dragon Bullet
Earth Release: Earth Flow River
Earth Release: Earth-Style Wall
Fire Release: Fire Dragon Bullet
Fire Release: Fire Dragon Flame Bullet
Roof Tile Shuriken
Shadow Clone Technique
Shuriken Shadow Clone Technique
Summoning Technique (Monkey King: Enma)
Telescope Technique
Hiruzen w/ Enma Beast Human Clone
Dynamic Marking
Fang Passing Fang
Fang Rotating Fang (Manga only) - Capable of crushing stone
Fang Wolf Fang - The power is great enough to dent a Rashōmon gate
Four Legs Technique
Human Beast Combination Transformation: Double-Headed Wolf
Passing Fang
Akimichi clan Butterfly Bullet Bombing
Calorie Control
Double Human Bullet Tank
Human Bullet Tank
Multi-Size Technique
Partial Multi-Size Technique
Spiked Human Bullet Tank
Super Multi-Size Technique
Super Open Hand Slap
Yamanaka Clan Mind Body Disturbance Technique
Mind Body Switch Technique
Mind Clone Switch Technique
Mind Disturbance Dance Performance Technique (Anime only)
Mind Puppet Switch Cursed Seal Technique
Yamanaka Clan Telepathy
Uchiha Clan Fire Release: Dragon Flame Release Song Technique
Fire Release: Great Dragon Fire Technique
Fire Release: Great Fireball Technique
Fire Release: Phoenix Sage Fire Technique
Fire Release: Phoenix Sage Flower Nail Crimson
Manipulating Windmill Triple Blades
VIZ PROOF.
"The sandaime, in particular, was a genius nicknamed the 'Professor,' and was called the strongest of all the Hokage".
the Sandaime was the strongest.
Databook 1, Page 117
The "Professor" revered as the strongest Hokage in history.
Databook 1, Page 119
"History's Strongest", "Genius", etc., there are many words that describe the Sandaime Hokage.
DB2 Pg 84
Konohamaru paragraph:
Konohamaru, descended from Sandaime, who was praised as history’s strongest Hokage.
"Hiruzen in his Prime was able to counter every Leaf Jutsu in Existance including all MS and EMS skills." DB2 Pg 84
Konohamaru paragraph:
Konohamaru, descended from Sandaime, who was praised as history’s strongest Hokage.
He who even holds the affectionate name of "Professor," being well-versed in every jutsu that exists in the village of Konohagakure
Lord Hiruzen is love. Lord Hiruzen is life. 
