Bolvar Fordragon. Arthas himself couldn't break him so I an sure he can handle the burden of controlling the army of the undead and keeping it from spreading like a wave of unguided killing power.
Cenarius came back with Malfurion Stormrage from the Emerald Dream as well..
For now. I can guarantee that even Bolvar will reach a breaking point some day, that helmet is basically the manifestation of pure malice and will corrupt anyone, no matter how strong, given enough time. Look at Arthas, a strong willed and dedicated man who fell simply because he became obsessed with saving his nation, which is a pure intention but led him to desperation and made him an easy target.
I imagine Bolvar struggles already with keeping that corruption at bay, eventually he will become so obsessed with containing the Scourge that he will be willing to do anything to do so, that's when he'll be vulnerable and that's when he'll break. We haven't seen the last of the Scourge, or maybe
we have, but Azeroth certainly hasn't.