1. Hashirama
2. Minato
3. Hiruzen / Tobirama
4. Tobirama / Hiruzen
5. Tsunade
We really can't rank them since we've seen nothing from Tobirama, and we only got to see a bit of what Hiruzen could do. And that was when he was frigging 70.
Still, I think that it's pretty clear that Hashirama is the strongest, and Tsunade is the weakest. If KCM is included as a part of Minato's abilities while alive, then I'd say that he's second.
Based on hype, Hiruzen should be extremely powerful. You don't just get called God of Shinobi for no reason. He was viewed as the strongest of the Gokage of his era, and he lived during the times of the Third Raikage and the Third Kazekage, who was considered to be the strongest Kazekage.
Tobirama... Well, on the plus side, he killed Izuna, who was supposedly on par with MS Madara. On the downside... Well, based on current facts, he got owned by 20 nobodies. Unless that's not true, and it was really something else that killed him, well, it takes away significantly from his stock.