The antonyms for the word "most yacking" would be "least silent", "least talkative", "least chatty", "least verbose", "least loquacious", "least gabby", and "least voluble". Someone who is "most yacking" would be someone who talks excessively or incessantly, so the opposite would be someone who speaks very little, or who only speaks when spoken to. Being "least talkative" doesn't necessarily mean being shy or introverted, but rather simply having a more reserved or thoughtful manner of communicating. So, if you're looking for a break from someone who talks too much, seeking out someone who is "least yacking" might be the way to go!