First hokage not only had wood style jutsu which was the most powerful form of ninjutsu, he also had instant seal free regeneration and was able to defeat madara and the nine tails simultaneously. At the current time, madara is crushing the 5 kages and a very weakened nine tails (only half its chakra) defeated the other 5 tailed beasts without much trouble. Therefore, Hashirama is stronger than the 5 kages and 5 tailed beasts all fighting him simultaneously. Minato could not defeat the nine-tails and thus had to seal away half its chakra killing himself in the process and he had much trouble fighting tobi. Therefore, even tho I love Minato, Hashirama would no doubt defeat him.