Who is the strongest Hokage? Who is actually the strongest Hokage? It would be such a simple question and answer if Kishimoto just gave us a direct answer. But instead, Kishimoto feeds us different information along the way as the story progresses.

Pre Chuunin Exam
Sarutobi was highlighted as the genius Shinobi. The manga stated that he quickly surpassed both the first and the second at a young age, and was widely known as the Professor. We also find out later that the first and second died in war, while the third and fourth sacrificed their lives to use the demon god. 

Chuunin exam era + beginning of Shippuden
Minato was considered the strongest Hokage. Recall when Orochimaru was using Edo Tensei to revive all dead Kages - Sarutobi was most afraid of the Fourth. For a while, everyone regarded the 4th as the strongest shinobi. I am not sure if this is because they never witnessed the first, but it seems evident that Minato was the powerhouse. Everyone kept saying - If the fourth was here, Orochimaru would be no problem, etc.

Late Shippuden when Tobi starts acting and Madara is revealed
Hashirama is the God of Shinobi. He is depicted as the strongest of all four, even recognized by Minato. He is the only one to resist Orochimaru's control. Madara Uchiha also regards everyone else, aside from Hashirama, as insignificant.

The thing that bothers me the most is how Sarutobi was not worried and was able to handle fighting both Hashirama and Tobirama. Minato was the source of his fear.

And then later on, Hashirama just seems like he is on another level. So I guess I have to redefine my question. Was Hashirama always the strongest Hokage, or did Kishimoto develop his strength later on? Or can this just not be determined?
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).