when it comes to the hokage im not even sure which is the strongest as they all have hype as much as others but minato seems to have more as kishi always uses him as the main person for naruto to surpass and if minato wasnt at least in the top 3 (im not sure which number i will put him) the why would kishi make him out to be one off the greatest shinobi in naruto as many characters have said know one can surpass him except naruto. with that said the 3rd said so him self its not the number of jutsu its your will to not give up which they all have but just because 1st, 2nd, and the 3rd have bigger jutsu than minato and tsunade doesnt mean they will instantly be better its about choseing the right jutsu for they job and the bigger the jutsu the more chakra they use so say hashirama spammed giant ass trees everywhere he would deplet his chakra way to fast and minato would just dodge them most likely but i doubt any hokage would be that reckless anyway to fight minato you need strategy more than power as if all it took was power than raikage would not lose to minato, but like with minato and the other hokage we have not seen the full battle powers, so if i was to list them it would have to be
hashirama
hiruzan/minato
tobirama
tsunade
this list is going of what we have seen not hype and im not sure if i can use this as canon but in naruto storm generation it shows a bit of the fight between the 1st and madara and the 1st used the same sealing tech as minato where he took the fox from madaras control so if thats to go of anything the 1st wife like kushina did for minato taught him uzumaki jutsu, or and the 1st did not take the fox and madara down alone as it was because of his wife who seal the fox so he did have help not to diminish the feats of the 1st but people keep saying it was a solo mission for him