The antonyms for the phrase "most third rate" would be terms like first-class, top-notch, superior, excellent, and outstanding. When something is considered third rate, it is generally regarded as low quality or inferior. By contrast, its antonyms amplify its value and signify exceptional quality. To describe something as first-class is to say it's top of the line, while top-notch signifies that it's of excellent quality. On the other hand, superior denotes a higher quality than average, and excellent means outstandingly good. Hence, these antonyms paint a picture that is quite different from the phrase "most third rate.