Umm... Half of his body was destroyed by the Tsuchikage's Dust Release.. Edo Tensei not only helps him recover from any injury but he doesn't feel tired or winded, on top of that he has been brought back stronger then he was in his prime and has the 1st hokages cells fused with him... so this new and improved "Edo Madara" is, from what we can tell the strongest, but don't overestimate the original Madara. basically, what i'm saying is Madara would have trouble in a 1v1 with any kage, but because of his "perks" gained by Kabuto via Edo Tensei, that is what is making him seem so strong..
Serpent made a good point. It's already been well established (considering Madara's injuries) that were it not fore Edo Tensei, Madara would've had his left arm obliterated, making him easier to defeat.
In addition, it's obvious that Madara has been heavily experimented on, so, in some sense, he isn't even human anymore. He's more like a freak of nature like Orochimaru and Kabuto.
Not to mention the fact that Madara was already defeated by the 1st, and there's another thread floating around here that make a great argument that the 3rd Hokage was the greatest ninja behind the Sage of the sixth path.
So, in conclusion, Edo Tensei Madara is on an epic level, but when did Edo Tensei ever count for anything? The real measure is "reality", or Madara when he was alive. So we don't know truly how strong he was. Edo Tensei is a revealing yet is, ultimately, an inaccurate indicator.
