"Best one can do" means the highest level of effort or achievement someone is capable of attaining. There are several synonyms for this expression including "maximum effort," "highest quality," "utmost ability," "top performance," and "optimum outcome." These phrases indicate that the individual has given their best possible effort or has achieved the best possible result within their ability. Other similar expressions include "personal best," "top-notch performance," "peak performance," and "maximum potential." In essence, these phrases all convey the idea that an individual has gone above and beyond their usual effort level and has achieved a level of success that they can be proud of.