This paper proposes an extensive investigation of temperature-to-digital converters (TDCs) figures-of-merit (FoMs): the main features of these systems are identified in order to determine the best FoM for achieving an effective TDC evaluation considering the desired performance characteristics. Multiple FoMs are presented, ranging from already established to newly defined ones, in order to obtain a picture of the TDC state-of-the-art with several angles and perspectives providing a complete overview of this framework.