According to research, the rarest DISC type is usually considered Conscientiousness (or "C" type). This type makes up a small percentage of the population and is organized, detail-oriented, and highly responsible.
However, it should also be noted that each DISC type has its unique strengths and weaknesses, and no one type is inherently better or worse than another. Therefore, it can also be argued that all DISC types are equally rare in their ways. Ultimately, valuing and appreciating the diversity of personalities within any team or workplace is essential.