Antonyms for the phrase "did one best" might include "performed poorly," "failed," "underachieved," "lackluster," or "mediocre." These antonyms suggest a lack of effort, skill, or dedication to a task, whereas "doing one's best" implies a commitment to putting forth one's maximum effort in order to achieve success. When someone does their best, they are likely to feel a sense of satisfaction and accomplishment, even if their efforts do not result in complete success. In contrast, those who do not do their best may experience frustration, disappointment, or even shame.