Antonyms for the word "most uncomplaining" would refer to someone who complains or expresses dissatisfaction frequently. This person can be described as discontented, unhappy, disgruntled, or dissatisfied. Other antonyms could include whining, grumbling, moaning, groaning, or griping. These words imply a continuous state of irritation or frustration, which contrasts the calm and accepting nature of the most uncomplaining person. The antonyms would apply to someone who is generally negative and pessimistic, and who struggles to see the positive aspects of any situation. In contrast, the most uncomplaining person is resilient and adaptable, accepting whatever life throws their way with grace and ease.