No Disrespect to Hiruzen, But...
I can only laugh at this part now:
In my opinion, the 3rd was one of the weakest, if not the weakest and least impressive of the Hokages. Each one of them has at least one unique thing about them which separates them from the rest. Hiruzen doesn’t have that.
Naruto is Naruto, I doubt I have to get into what diversifies him.
Kakashi is the creator of the Chidori, and is the only ninja in history capable of using both the Chidori and the Rasengan.
Tsunade’s Byakugou Seal enables her to heal fatal injuries inflicted upon herself without using hand seals; she survived getting cut in half.
Minato is the creator of the Rasengan, he perfected the Flying Thunder God technique, and was known as “Konoha’s Yellow Flash”, the fastest shinobi in existence.
Tobirama created the Flying Thunder God technique, created the Reanimation jutsu, and has overwhelming levels of chakra.
And Hashirama is known as “The God of Shinobi”, ‘nuff said.
In comparison to the rest of them, Hiruzen doesn’t really have anything which sets him apart. And if there were ever any doubts regarding who the strongest Hokage was, chapter 620 made it perfectly clear.
After Sasuke made his intentions clear, Tobirama didn’t take too kindly to them, and made those present feel the overwhelming force of his chakra:
It was enough to disorientate everyone in the room (apart from one), and cause Hiruzen to panic:
However, Hashirama was completely unfazed, and proceeded to show his little brother the true meaning of overwhelming chakra levels:
The sheer force of Hashirama’s chakra caused Tobirama to sweat, and the elder brother proceeded to give the 2nd Hokage demands, as if to remind him of his place:
And if that weren’t enough, Hashirama subsequently laughed it off as if that overwhelming demonstration of power was nothing to him. Whereas it caused everyone else present to fear for their lives, with Hiruzen actually admitting that the force of Hashirama’s chakra scared him:
This also wasn’t the first time Hashirama had completely made Tobirama his bitch using only the sheer force of his overwhelming chakra. Ages ago, when the Uchiha and Senju clans were still actively feuding, Tobirama opted to end Madara’s life when he was given the opportunity, but Hashirama refused to let him, much to the perplexion of his younger brother:
So when Tobirama persisted, Hashirama pierced a hole through Tobirama’s very soul with his glare alone, while spiking his chakra to unimaginable levels, daring his brother to proceed:
And his brother, intimidated beyond belief, duly relented:
This is the power of Hashirama Senju.
Furthermore, Hashirama was the only one capable of moving around freely and of his own accord, despite being under the Reanimation jutsu; Orochimaru was unable to control “The God of Shinobi”:
So yeah, Hiruzen isn’t even in the same league. if any Hokage deserves to be described as “The strongest of all”, it’s Hashirama Senju.