I'd give it to King as well. Zoro was pushed to his limits there. Granted he didn't know or seemed to know how to use Haoshoku embed attacks until thinking back on his time as a kid and piecing things together... But he was overall getting outworked by King. Against Lucci, they seemed to be in a stalemate without Zoro being threatened that much besides tiring himself while he was not using it, while King was actively hurting him iirc.
Lucci still taking Zoro's Haoshoku attack and standing speaks to his outstanding durability, which may prove to be a dragging battle to king, but considering Jimbei stated Zoro had already won, I think Lucci would be able to drag and tire King, but at the end, King would give him the merciful blow to put him to sleep, mid-high difficulties, but not extremely high or anything.