The antonym for "most nonplussed" would be "most composed" or "most unfazed." These terms suggest a state of calm and collectedness, indicating that the individual is not perplexed or unsure of what to do. When someone is most composed, they are able to think rationally and react in a controlled manner even in stressful situations. On the other hand, someone who is most nonplussed is easily confounded and discombobulated, lacking the ability to react properly. Using the antonym for "most nonplussed" can indicate a sense of stability and confidence, benefiting individuals in both personal and professional settings.