Mukuro. It is largely overlooked but after Raizen weakened over the course of centuries, Mukuro's youki level was overall the highest of the 3 Kings. That being said, Yusuke lost to Yomi. Almost certainly he would lose to Mukuro as well, especially given the nature of her primary ability. Hiei only barely managed because of his immense speed. Yusuke who should be a little slower would probably have a harder time even if he is slightly stronger than Hiei.
Actually Mukuro was barely at half strength in the tournament. Her power comes from her frame of mind. As seen when she became angry with Hiei, she blitzed him and badly injured him with a single punch. So she could beat Yusuke pretty easily if she is angry.