Who is the strongest Hokage?
During the fight between Sarutobi Hiruzen and Edo Tensei Hokage brothers, the 1st and 2nd Hokage were not able to use their full power, because, Orochimaru hadn't perfected the jutsu at that time.

Also, Hiruzen was not afraid of Minato alone, but he was afraid that he wont be able handle all three of them at once.

In my personal opinion, Hashirama Senju is considered the strongest Hokage because of his abilities like Sage Mode, extraordinary life force, Wood Release techniques, impressive clones, great stamina (many people have tries to replicate it, like Orochimaru, Madara Uchiha, Zetsu, Obito Uchiha).