It's either Madara, the masked man, or Hiruzen. It's difficult to conclude which of these are the strongest. However there's evidence for why each and everyone of them would be considered the strongest.
But you should probably take a look at the Naruto tier list in the Naruto discussion thread. Because Q99 has is probably the most knowledgeable member on the topic.
Madara is the strongest as far as feats are concerned.
We don't know how powerful Tobi is, but we know that in his current state he might rival and even surpass Madara in terms of power -- it wouldn't come as a shock if that was the case.
Hizuren was praised as the most powerful hokage of all time, putting him above both Hashirama and Minato. In fact he even took out Hashirama and Tobirama when Orochimaru reanimated them with Edo tensei. But Madara is clearly stronger than Hashirama at this point considering how he has his power.
The answer is. We don't know, and there's a possibility that we never will.
"To all visitors from Transylvania looking for the head of Voivode Dracula: Yes, we have it. Yes, he's dead. No, you cannot see it. No, he will not return and invade you again. It has been over thirty years, please stop pestering us."
Hashirama and Minato were stronger than Hiruzen, when Hashirama was revived Kishi did not put his full power, and even without the power, Hashirama got to be a great problem to Hiruzen
"To all visitors from Transylvania looking for the head of Voivode Dracula: Yes, we have it. Yes, he's dead. No, you cannot see it. No, he will not return and invade you again. It has been over thirty years, please stop pestering us."
Surprisingly, I'd say Naruto with his partnership with Kurama. At his current power they could plow through Madara's trees and are strong enough to break his Susano'o too.
Current Tobi with Paths would stomp Minato. And there's the likelihood that the Rinnegan does not go blind when Izanagi is used.
So yeah, he's easily up there.
__________________
"To all visitors from Transylvania looking for the head of Voivode Dracula: Yes, we have it. Yes, he's dead. No, you cannot see it. No, he will not return and invade you again. It has been over thirty years, please stop pestering us."
Izanagi needs Uchiha & Senju DNA, and the Sage of the Six Paths had the Rinnegan, not the Sharingan. So Tobi can still use Izanagi or the version the Sage used, which was much more powerful.
__________________
"To all visitors from Transylvania looking for the head of Voivode Dracula: Yes, we have it. Yes, he's dead. No, you cannot see it. No, he will not return and invade you again. It has been over thirty years, please stop pestering us."
"To all visitors from Transylvania looking for the head of Voivode Dracula: Yes, we have it. Yes, he's dead. No, you cannot see it. No, he will not return and invade you again. It has been over thirty years, please stop pestering us."
Dunno, but seeing as his Izanagi lasted for 10 minutes, he definitely has Senju DNA.
__________________
"To all visitors from Transylvania looking for the head of Voivode Dracula: Yes, we have it. Yes, he's dead. No, you cannot see it. No, he will not return and invade you again. It has been over thirty years, please stop pestering us."