Either Magneto or Beast. I don't know how they're going to end up redeeming Beast or rerouting him without going all the way to, at very latest, Astonishing X-Men. He's full on fell to the depths of villainy, a slow slide about 10 years ago, but the last year or two he's just rocketed down the abyss.
Magneto's much more complicated. His worst act was probably during Fatal Attractions with the global EMP wave - think about people in airplanes, hooked up to life support machines or otherwise medically dependent on electronics, pretty much all dead. No telling how many millions that covers. Before that, he'd done some really bad stuff but his storyline cycled between villainy and redemption. After Fatal Attractions I don't feel like there was a path to come back from that, and was full on against him joining the X-Men during the Utopia era. That said, take away the Fatal Attractions storyline and I could see much of what he's done in the last 15 years mostly redeeming him.
I'm not counting the Planet X storyline as after the retcons and retconning those, it doesn't seem like that was the real Magneto. What he did to New York certainly was not in character, given what he went through in the Holocaust.
So for Magneto it's mainly because of one act (two if you do count Planet X), for which he's trying to atone. I still don't think he should have been given the chance, but it is what it is, I don't write the books. Beast is enjoying this slide into what he's become, and much of what he has done has been hands on. That makes it a bit more personally twisted than many of Magneto's acts, many of which were done from afar.