The word "most baritone" is a superlative form of the adjective "baritone," which is commonly used to describe a deep, resonant voice or musical instrument. The antonym for "most baritone" would be "most soprano," which refers to a higher-pitched, lighter-toned voice or instrument. Other antonyms might include "tenor," which is a moderately high-pitched voice or instrument, and "alto," which is a lower-pitched voice or instrument. Depending on the context, other antonyms could include "thin," "reedy," or "flat" if referring to a lack of depth or resonance in the voice or instrument.