The antonyms for the word "most amateur" are "most professional" or "most expert." Choosing to describe someone as professional implies that they have a high level of expertise, experience, and skill in a particular field. On the other hand, an expert is someone who possesses a deep understanding of a subject matter and has demonstrated proficiency through their work. These two antonyms stand in opposition to the term "amateur," which typically denotes a beginner or someone who is inexperienced in a particular field or activity. Therefore, when describing someone who is highly skilled or experienced, it's best to use the antonyms "most professional" or "most expert.