The antonyms of "first-class" include words such as mediocre, average, inferior, low-quality, poor, and cheap. These words describe something that is substandard or of lower quality compared to something that is first-class. For instance, a second-rate hotel may provide inferior quality services compared to a five-star hotel, which is first-class. Similarly, a budget airline may provide cheap tickets but lack the luxurious amenities and services that a first-class airline would offer. In summary, antonyms of "first-class" describe things that do not meet the highest standards and are of lower quality.