Well, this is down to how you define "strong."
In Pure physical terms, there are a number of characters who can lift multiple tonnes, like Spider-Man and Wolverine. However, it is a challenge, from my perspective between the Hulk and Thor; both have surreal levels of strength.
In terms of mental, then it is probably Professor Charles Xavier, being considered the worlds greatest telepath, though Emma Frost is comprable.
In terms of human spirit, then you have to consider Tony Stark as the probable winner to this. He went through being kidnapped, massively injured, creating an armor to escape captivity, and had two alcoholic spells. He pulled himself out of those without people telling him to, or outside help. So, he himself could be deifned as "iron Man," as opposed to the armor suit he wears.
I hope this helps to further the debate!