Antonyms for the word "most costless" could be "most expensive", "priciest", "high-priced", "costly", "lavish", "extravagant", "premium", "dear", "sumptuous", "luxurious", "pricey", "valuable", "invaluable", and "precious". All these words indicate something that is expensive or valuable. When we think about something that is costly, we automatically associate it with high quality or rarity. On the other hand, something that is costless might not have any value or quality. Therefore, antonyms for the word "most costless" can range from basic synonyms of "expensive" to words that reflect a high level of quality, luxury, and rarity.