The word "most spleenful" is not commonly used in English language, and hence, there are no exact antonyms available for it. However, if we break down the word into its parts, we can find antonyms for each individual word. "Most" can have antonyms like least, fewest, or minimal, depending on the context. "Spleenful" can mean spiteful or malicious, so its antonyms could include friendly, kind, or benevolent. So, if we were to interpret the phrase "most spleenful" to mean the most full of malice, its antonym could be something along the lines of least spiteful or most benevolent.