The term "most retentive" implies the ability to retain or remember information. The antonyms for this word would, therefore, be those that imply the opposite. Some words that could be used as antonyms for "most retentive" include forgetful, unretentive, unfocused, and unfocused. Other possible antonyms may include absent-minded, inattentive, absent, scatterbrained, or distracted. These words suggest that the person or thing in question is prone to forgetting, losing focus, or being distracted from important information. It's important to note that antonyms can have different nuances, depending on the context of the sentence, but always indicate the opposite meaning of the main word.