The phrase "most graying" is typically used to describe someone or something that is experiencing a significant amount of gray hair. There are a number of synonyms that can be used to describe this state, including "graying most rapidly," "most silver," "most salt-and-pepper," "most hoary," and "most grizzled." Other possible synonyms might include "most aged," "most mature," "most weathered," or "most seasoned." Ultimately, the choice of word will depend on the context in which it is used and the desired tone or connotation.