If something is first-class, it's the very best. First-class seats on an airplane are the roomiest and most comfortable—and the most expensive.
This adjective is perfect for describing the highest-quality version of something, from first-class stamps that get your letter to its destination faster to a first-class detective who relentlessly follows every single clue until the mystery is solved. First-class orchestras are highly accomplished, and a first-class moving company is unlikely to break your dishes while transporting them to your new house.