There are several synonyms for the term "least effective" that can be used depending on the context. For instance, if you're referring to a weak or pathetic performance, synonyms such as ineffectual or feeble are appropriate. Alternatively, if you're describing someone or something that is not achieving the intended results, unsatisfactory, inefficient, or ineffective could be used synonymously. Additionally, the words inadequate, substandard, and weak can be employed as synonyms depending on the situation. Ultimately, it's important to select the right synonym that accurately conveys the intended meaning and context.