Only two characters in the entire Naruto verse have been given the title of "God of shinobi." The first one is Hashirama Senju (the first Hokage), who was the founder of the Hidden Leaf village and the original user of Wood-style Jutsu. The second is his protege, the Sandaime Hokage of Konoha, Hiruzen Sarutobhi.
His mastery over all the Jutsu in Konoha was unparalleled, earning him the moniker of The Professor. But sadly, even after achieving the position of Hokage at the youngest age and holding the chair much longer than any predecessor or successor, people still believe that he wasn't as strong as is made out to be. So, here's an article that will attempt to explore why he was the most underrated Hokage stating seven reasons.
From being mentored by the two Hokages to being the mentor of the Legendary Sannin, here's what makes Hiruzen legendary
1) Mastery over all five natures of chakra
He was one of the select few shinobi to master all five natures of chakra - a feat accomplished by only a select few shinobi in the entire Naruto franchise. This included the first Hokage (Hashirama Senju), the second Hokage (Tobirama Senju), and Hiruzen's protege, Orochimaru.
Since he was from the Sarutobhi clan, he was much better suited for fire-style techniques. However, he made no mistake when it came to other forms of releases. Whether it be wind, water, earth, or lightning, the potency of his techniques was nothing to be sneezed at. Any fan could attest to this claim if they have seen the old Hiruzen fight with Orochimaru.
2) The Legendary Sanin were his disciples
It's no rocket science to understand that a good teacher always tends to have good students, but the word good doesn't come close to describing the students that he trained. Even the weakest of the trio was strong enough to hold the title of Hokage, and the other two were feared by nations.
The toad sage, who was arguably the most popular mentor in the whole Naruto series, was one of Hiruzen's students. He went on to train two Hokage (Minato Namikaze and his son, Naruto Uzumaki) and three Hidden Rain war orphans who would later form the Akatsuki.
Even the most crooked of his student was such a feared shinobi that he took over an entire village and slaughtered another's Kage single-handedly. He also trained the lifelong rival of Naruto and made him into a man who was feared by nations.
3) Knowledge of all the ninja arts in Konoha
A shinobi is as powerful as the number of ninja arts he possesses because, in a one-on-one battle, these techniques are the only thing that can make one win. Now, consider a shinobi who knows how to use each and every technique present in Konoha apart from Kekkei Genkai (techniques that require a specific bloodline) and clan-specific hidden techniques.
Rasengan, eight inner gates, Chidori, nothing is off the table for him. Just imagine if a weak and old Hiruzen can take on a shinobi of Orochimaru's caliber with him being aided by Edo Tensei of the first and second Hokage, what the prime Hiruzen would be like?
4) Possesses one of the strongest summonses in the Naruto series
Enma may very well be one the strongest summoning animals in the Naruto franchise, so much so that Orochimaru became agitated when the old Hiruzen tried to summon him against his reanimation Jutsu. The monkey king's body is mostly impervious to physical attacks, and his shape-shift ability makes it all the more troublesome for the opponents to deal with.
The monkey king Enma was so strong that it took on both a reanimated Hasirama Senju and Tobirama Senju simultaneously and was still able to turn the tide of battle in Hiruzen's favor. Interestingly, unlike his master, Enma was not as emotional as Hiruzen and was ready to eliminate Orochimaru once knowing his unethical research.
5) Mentored by both the First and Second Hokage
The third Hokage Hiruzen Sarutobi was the only person in Naruto who trained under the tutelage of both Hashirama (First Hokage) and Tobirama Senju (Second Hokage). He was the son of Sasuke Sarutobi, a shinobi so skilled and highly respected that Fugaku Uchiha named his second-born after him.
Hiruzen belonged to the Sarutobi, clan which was one of the pioneering clans to inhabit the Village Hidden in the Leaves. Being an efficient user of fire-style jutsu from the get-go, Hiruzen was quite a prodigy to the point that even a man like Shimura Danzo, who hated him and his principles, had acknowledged how fluent he was with every jutsu he tried.
6) An inspirational leader and a keen judge of hidden potential and character
During the second Shinobi War, when the small troop of Second Hokage was chased by the special Kinkaku and Ginkaku unit of the Hidden Cloud Village, Hiruzen was the first one to answer the call when Tobirama asked for one of the team members to be the decoy knowing full well that the one doing this task will be dead for sure.
There have been many instances where Hiruzen has been able to discern the hidden potential of people, even in the face of failure. For instance, during the Chunin exams, he was able to discern the hidden potential of Shikamaru Nara even with his laid-back attitude, so much so that he was the only one who passed the exams on the recommendation of the Third Hokage, and the rest was history.
7) His age of retirement
Sarutobi was the youngest Hokage who maintained that seat for the longest duration of all the Hokages. He was not from the era where peace and small escalations were the new normal but from the era where conflicts and death were everyday news.
He was the only Hokage that witnessed all of the Shinobi world war, including the fourth Shinobi world war, even though it was in his reanimated body. Thus, surviving this long in the world where men or especially leaders die young has to be the biggest accomplishment in itself.