King is clearly the strongest of all opm characters besides, perhaps, Saitama. King's power is such that reality bends to accommodate his victory. His battle prowess can be represented as an ? since he will never actually reach combat but always defeat his foes.
You see, the "limit" King broke was from his own desire to avoid social contact so he could play games like Doki Doki Sisters. That is what gives King his incredible aura of intimidation which tells foes "Stay Away" and "Leave Alone". It's actually telling that, without Saitama, King would have no friends. Only Saitama and other really strong characters can resist King's intimidation aura.
King's friendship with Saitama only makes his power stronger; the chances that King will have to act in order to defeat his enemies is basically zero at this point. So really the only reason he isn't in the conversation as the the strongest high tier s class is that he is beyond high tier; God tier even.