According to the databook, Hiruzen is the strongest of the Hokage. I don't care what any character has to say, the databook>characters. Now that that's out of the way, I'm going to attempt to put an end to the Hashi vs Hiruzen debate. I'll probably fail though, but we'll see....
I would like to point out that being the strongest Hokage doesn't mean that he can nessecarily beat Harishima in a one on one fight. Allow me to explain.
Hiruzen, quite frankly, was the strongest Hokage for obvious reasons. He was nicknamed the professor and at one point knew every jutsu that existed in the Leaf. That would mean that he knew WAY more jutsu than Harishima did. He knew all jutsu from Harishima's lifetime and then some, basically 3 Hokage generations worth of knowledge.
In part 1, the idea that strength=justu mastered was heavily stressed through the main villian Orochimaru. In fact, I think the reason why Oro felt that being stronger=learning more jutsu is because his sensei, Hiruzen!, was considered the strongest because he knew more jutsu than anyone.
Now the reason I say that having "superlative strength", as Iruka put it, doesn't mean you can always beat your opponent, is because we've seen countless fights where weaker enemies won for reasons such as fighting style advantage, intelligence, or simply being able to hit the right jutsu at the right time. I have 2 examples for you:
Example 1: Rock Lee vs Sasuke.
When Lee fought Sasuke during the chunin exam extrance, Lee OWNED him. Now, clearly he wasn't stronger than Sasuke by any means. Sasuke had the sharingan, the chakra and the knowledge. But, as we learned though that chapter, Lee was simply too fast for Sasuke to avoid his attacks, even though Sasuke could see them clearly. Lee, a fighter with only Taijutsu, defeated a stronger Sasuke.
Example 2: 3rd Hokage vs Naruto.
Now this wasn't a fight, but Naruto still took down Hiruzen with a pretty pathetic jutsu. We ALL know Naruto was not stronger than Hiruzen in part 1, but this is exactly why I say that being stronger does not mean you can't get caught off guard by the right jutsu. Naruto took down Hiruzen with the sexy no jutsu, yes, SEXY NO JUTSU.
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I know some of you are going "this doesn't mean anything" but you clearly see Hiruzen laying unconscious on the ground with a nosebleed. He's the 3rd Hokage, yet he got caught off guard by a jutsu like this. That's because it's not always about being stronger, but having the right jutsu in your arsenal.
Plus, this jutsu doesn't work on every male in the manga, as you see here when Konohamaru trys to use it on his sensei:
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As silly as it sounds, This weak sensei avoided a jutsu that the 3rd Hokage fell prey to. But we all know Hiruzen isn't weak.
So there you have it, sorry it's so long, but I learned that you have really explain things on NB. Also, since Hiruzen knew more jutsu than Harishima, it's very possible he knew a jutsu capable of beating Harishima. No one really knows
