Let's all just agreed that until we have full disclosured of all the Hokage Techniques, and powers we can never know which Hokages are ranks where.
I personally believe Hashirama was the strongest Hokage, and the Strongest Shinobi not name Sage Of The Six Path.The problems is I can't prove it and I don't know all Hashirama jutsus and his fighting style(skills).Being a wood element users, Hashirama has mastery of both earth and water element jutsus but we haven't seen either.He also was a genjutsu type, but we have only seen one genjutsu from him, and we know he has to have more.
Tobirama was the most powerful water jutsu users, has a S/T ninjutsu that is a complete unknown, and was the creator of the Impure world jutsu.And for those who said he didn't master it, he obviously use it for so many of the resurrected shinobi to know it.Meaning he can use it in battle.
Hiruzen was said to be the God Of Shinobis and he was a student of the first and second, but we don't know anything about his skills in his prime.He has shown fire and earth element jutsu and a skill with shadow clones.
Minato has the rasengan, toad summonings, FTG, and sealing jutsus.But we don't know his chakra nature.Remember Kakashi said that Minato originally evented the rasengan to mix with his nature affinity.So that means Minato has nature element jutsus as well, and possibly more than one.
Tsunade, who is always disrespect has Monsterous strength, Slug summonings, sealing jutsu, Creation Rebirth, and a persumed lighting chakra nature which we have yet to see her use beside in her battle with Kabuto.
With all that said, how can we rank all the Hokages when we have so little information on all of them?