The term "depthless" refers to something that is without depth or shallow. Therefore, antonyms for "most depthless" would be words that describe depth and profundity. Synonyms for "depth" could include profundity, thickness, substance, profundity, and complexity. Adjectives that convey depth could include deep, profound, thoughtful, complex, and weighty. On the contrary, words that could be antonyms for "most depthless" could include shallow, superficial, lightweight, simple, superficial, and insubstantial. In summary, the antonyms for "most depthless" would describe something with substance, complexity, or depth.