Sanhedrin 18: Limitations on Who Can Be a Judge
Chapter 2! With a mishnah about various legalities with regard to a high priest (kohen gadol), including him not being a judge nor judged. And issues of yibum/halitzah. Plus, other roles and their various legalities (such as a king). Also, the Gemara on the mishnah, delving into the limitations on who can be a judge - beginning with the kohen gadol. Plus, some discussion about the overlapping spheres of authority (that should not overlap) and the roles of the priests.