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.
Originally posted by TheAuraAngel
On that particular tier list, Madara is the strongest character. Tobi/Naruto take second.
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.