Looks can be deceiving, but make no mistake, it's not Madara that is the Invincible Immortal--no, it's Umino Iruka. Despite being a mild mannered teacher, Iruka has displayed an incredible fortitude that has yet to be replicated.
Being stabbed multiple times with Kunai while protecting Naruto, Iruka didn't falter for a second and was still able to keep going with little effort. As if it weren't enough, on top of being wounded by the Super Genius Mizuki with the Kunais, he was also stabbed once again in the back by a giant Shuriken that easily weighed a few hundred pounds as it was made of metal. However, it was nothing to Iruka who had leapt in front of Naruto to protect the boy from being sliced in half. Instead, the shuriken barely made it past his skin and stuck into Iruka's back. A lesser men would have been cut in two and died. In fact, lesser men have died, like Yahiko for example, from much smaller weapons. Despite being trained by the legendary Sannin Jiraiya and the leader of Akastuki, all it took to bring him down to his knees was one single kunai to the stomach.
Even the great Jiraiya himself succumbed to wounds that would have barely nicked Iruka's invincible iron like skin. Pain's black rods likely wouldn't have made it past the dense fibrous tissue that make up his muscles, just like that giant shuriken couldn't even touch his spine. Jiraiya would probably be alive today if it'd been Iruka who'd fought against Pain, instead of him.
I guess it's no wonder that someone who could survive the Kyubi attack on Konoha while still only child, would grow up to be the true Invincible Immortal.