Hiruzen is overrated more so than underrated.
Before the manga made it perfectly clear that Hashirama is on a completely different tier to the rest of the hokage's, i was supporting the Hiruzen movement. I was arguing in his favour, but even i had doubts in my mind and i ams sure those who supported him also did.
My main point for supporting hiruzen was because i hate how this manga has become a senju/uchiha nonsense, that is more or less the reason why i was supporting him.
Anyway as i said he is overrated more than anything and it is simple.
This 'prime' hiruzen is becoming fanfic, no different than healthy itachi/nagato and recently alive kcm minato. Yes i believe the nagato that faced hanzo was healthy, but at the same time believe itachi wasn't born ill so he was healthy at some time in his life. But like prime hiruzen they do exist, but the problem is we cannot gauge them or should i say how strong they were.
Prime hiruzen has 0 feats and empty hype.
Some have tried to claim him linking obito's jutsu's to onoki's jinton means that he faced onoki.
WHile it may be true, that still isn't prime hiruzen. If they were to meet it would have been during the 3rd great way, but hiruzen was still old and not in his prime at that time. Personally i believe the reason he was able to link obito's jutsu with jinton is because as a kage you'd know at the bare minimum the basic abilities of the other kage's. After all you will be having meetings and what not. We saw at the recent 5 kage's meeting that apart from gaara who was the youngest the rest had a rough idea of each other's personalities and abilities.
If we were to claim he faced onoki as his greatest feat to demonstrate 'prime hiruzen', then why is onoki still alive. Granted onoki is far from fodder, but a prime hiruzen with his hype should have put him away, especially with him being the enemy's leader i doubt hiruzen would have let him live, as in a time of war cutting off the head of an alliance is a strategy something i cannot see him letting escape.
If anything the proposal of him fighting onoki shits on his hype and if anything boosts onoki's
His only hype standing is professor and we got a glimpse of that when he easily analyzed current obito's attack, but that isn't as great a feat as shinobi's such as itachi/kakashi have done something similar.
His power is stemmed from his array of justus, but also vast knowledge on genjutsu and taijutsu that he supposedly knew and that is about it. I do however believe that just like how the sannins summons vastly improve their powers and ninjutsu variations i believe the same can be said about hiruzen. He might have some monkey based ninjutsu just like how oro/jiraiya have variations.
So his vast knowledge and expertise along with his summon imo the best animal based summon makes him something special. How special, personally i believe he is around jiraiya/oro level. Strong but not there yet.
I am more of a feats rather than hype guy because majority of hype are tainted. By that i mean, character bias, character opinion, misconceptions, false statements, etc. There are only but a few true hype, therefore i like to stick to feats unless i believe a hype in untainted.
Currently Hiruzen based on feats is the weakest.