The word "goddamnedest" is a highly colloquial and somewhat vulgar term that is often used to express a strong sense of disbelief or amazement. While there are no perfect synonyms for this particular word, some possible alternatives might include phrases like "unbelievable," "incredible," "mind-boggling," "astonishing," or "amazing." Other possible synonyms might include terms like "shocking," "awe-inspiring," "jaw-dropping," or "flabbergasting." Ultimately, the best synonym for "goddamnedest" is likely to depend on the context in which it is being used, as well as the speaker or writer's particular tone and attitude.