The phrase "haddest effect" implies a positive outcome or impact on something. Therefore, the antonyms for this phrase would refer to negative or adverse consequences. Words such as "ineffective," "inefficient," "inept," "useless," and "unproductive" convey the opposite meaning of "haddest effect" and suggest a lack of success or accomplishment. Additionally, terms like "harmful," "damaging," "detrimental," and "disastrous" describe negative effects that can cause significant harm or destruction. When seeking to express the opposite of "haddest effect," it is essential to consider the degree of impact and the overall consequences of the action or event in question.