The antonyms for the word "most reactive" are "least reactive," "inert," and "unresponsive." When an element or molecule is least reactive, it has a low tendency to participate in chemical reactions. In chemistry, inert substances such as noble gases are used as protective or insulating materials. These substances do not readily enter into chemical reactions due to their stable electron configurations. On the other hand, the term unresponsive suggests a total lack of reaction or sensitivity to external stimuli. In summary, the antonyms for "most reactive" indicate the absence of chemical activity, stability, or insensitivity, depending on the context.