Hard to say, I guess I'd need a clearer definition of what the "moral center" actually means, but in my opinion, it certainly isn't Legacy. The existance of Iron Legacy shows me that Legacy is far too unbalanced and morally fragile to be considered the moral center of SotM. If the death of his daughter is all it takes to make him go full super-villain, than that just goes to show that perhaps his dauntless morality is not nearly as durable as it first seems. Honestly, I'd either go with Omnitron-X as the most moral person in the story I've seen. OX is willing to sacrifice himself to save the lives of humans, and has made it his goal to minimize the suffering of all people everywhere, going so far as to attempt to destroy his past self to undo the damage he had done before. He is, in my opinion, the most self-less character in SotM.
Also interesting morally is Mr. Fixer, who shut down his martial arts dojo in an attempt to protect his students, only to finally begin fighting back later in life. Just as Legacy's decent into evil proves how fragile and weak our morality can be, Mr. Fixer proves that morality is not black and white, and that sometimes there is no good option. Sometimes, fighting just makes things worse, and sometimes, when the world keeps pushing you, you have to push back. Mr. Fixer to me shows the reality of the world. Legacy is clean and wholesome, but clearly not as perfect as he seems at first glance, while Mr. Fixer seems more real to me.
The problem with this question is that morality is relative. Sometimes we need someone who will uphold our ideals, who will stand as a beacon of hope, like Legacy. Sometimes we need someone who will never compromise, who refuses to bow to adversity, like Wraith or Fanatic. Sometimes we need a realist, one who does the best they can with what they have, like Fixer. Sometimes we need someone willing to sacrifice everything to put things right, like Omnitron X. Sometimes we need someone calm and careful, patient enough to find the right solution, like The Scholar. And sadly, sometimes we need someone who'll get things done no matter the cost, who'll get rid of the bad guys even if it means putting them down for good, like Expatriette.
My point is, right and wrong depends on context, and therefore it's hard to say who is the 'moral center'.