The antonyms for the phrase "most unchallenged" can vary depending on the context in which it is being used. If the phrase is used to describe a situation or decision, then antonyms could include "hotly contested," "controversial," or "disputed." On the other hand, if the phrase pertains to a person or entity, antonyms could include "embattled," "challenged," or "compromised." In general, antonyms for "most unchallenged" would be words that imply some level of opposition or adversity, rather than a lack thereof.