"This Susanoo is destruction-incarnate, everyone who has seen it, has died."
I suppose Madara was right about everyone who has seen it has died, but if Tsunade heals the kage, then it will no longer be true.
"Yata no Kagami for defense, Totsuka no Tsurugi for offense, he is invincible".
The actual weilder of the tools also acknowledges that every move has a weakness. For the record, Itachi is as smart as Madara is cocky. In this case it's space time in general, and both versions of kamui in particular. They ignore defence and hit the target anyways. Also, the shield is a shield and not armor. It doesn't protect the entire susanoo, so the susanoo would still be vulnerable to attacks from behind. Also, dark swamp and other attacks from below should be able to best the user. Poison gas might also be able to work. I doubt susanoo is air tight, or else the user would suffocate.
"I told you not to open your mouth so easily! You're just words and no actions, what someone like you says is worth nothing!"
I think this is the reason Kakashi rarely makes promises anymore. No matter what he does, something always goes horribly wrong.
"He was known as the God of shinobi!"
Yes, Kishi decided to change his mind here, but going from what we've seen and you said, Hiruzen is the stronger Hokage. Also, I don't think Madara was active anymore as a shinobi during his prime to witness it.
Edo Madara is the strongest being!
Probably, on account of being immortal. BM Naruto is more powerful than he is, but has no answer for his immortality and the fact that he now packs Hashi's anti-bijuu techs doesn't help either. There are others who can force him to regenerate, but they hit the same problem. The closest things I can think of for counters are Itachi(totsuka), Obito/kakashi(extremely hard to dodge kamui), and Minato (speed blitz with reaper death seal, and call it a draw). Maybe Hiruzen for the same reason as Minato, but lower speed hurts him. If Madara is around long enough to learn the rest of his rinnegan arsenal and becomes the juubi jinchuriki, they're just plain screwed.