Piece de resistance is a French term (circa 1839), translated into English literally as "piece of resistance", referring to the best part or feature of something (as in a meal), a showpiece, or highlight. It can be thought of as the portion of a creation which defies (i.e. "resists") orthodox or common conventions and practices, thereby making the whole of the creation unique and special. The phrase gives the sense that the referred-to element is the defining essence of the whole, that part that makes it memorable or gives it its unique character.
e.g. ? Even with a diverse movie and television portfolio, Burnett's piece de resistance will likely forever be The Carol Burnett Show.
Originally, the piece de resistance was the most substantial dish in a meal, but now the term generally refers to quality, not quantity.