We dont really know because we've never seen the 1st 2nd or 3rd fight in their prime. I know that minato stomped all over madara in a few seconds, while also managing to protect his wife and son causing madara to flee.
IMO Flying Thunder God is one of the most powerful techniques ever created, it allows minato to take out fleets of over 50 ninja in about a second. His use of it is technically sound and he has no hesitation to kill if necessary, i think its hard ot argue against him being the strongest. However like i said, we need to see the other hokage in action.
We know the 2nd had some kind of space/time jutsu but we dont know what it was, and we know he had IWR but we also know that orochimaru improved it, and kabuto improved it further after that, so we dont know the extent at which he could use the jutsu. Harashima is most famous for his wood element, which on yamato isnt that impressive. Though we know that the 1st far surpasses yamatos wood release abilities.
The 3rd masterd all the known hidden leaf jutsu and is hailed as the God of Ninja, but he believed minato to be stronger than he was, whether or not that is true is undetermined.
People don't give tsunade enough credit
im not saying she is the strongest, but you have to consider that she IS AS STRONG AS jiraiya and orochimaru, there is a reason kishi grouped them as the 3 sannin. and i would guess jiraiya isnt that much weaker than minato.
