the most powerful hokage must have been the first, Hashirama Senju. due to the fact that he had wood release techniques which are very powerful he also had the contract seal which would make summon's useless. He held advanced ability in taijutsu with prodigious speed and strength. and he also had mastery of genjutsu as shown with his bringer of darkness technique. he had such mastery of medical ninjutsu that he could use jutsus of far greater magnitude than tsunade(who is the greatest medical ninja in the current era) with out forming hand seals. so if you think about it he had a massive collection of ninjutsu and he was also a master of earth and water elements for the wood release, he could use taijutsu effectively and kenjutsu(swordsmanship), he had genjutsu mastery and also effective jutsus against summons then on top of that he has medical jutsus that can heal himself while in combat. and extreme control over his chakra becuase it is needed for medical jutsu so none of his chakra is wasted. people should really not just chose minato just cause he created the rasengan and he can use the flying thunder god jutsu. Just cause hashirama is terrible in the video game doesn't mean its is really like that.