On the which council did Joseph of Arimathea sit? Mark 15:43
Joseph of Arimathea, a prominent member of the Council, who was himself waiting for the kingdom of God, went boldly to Pilate and asked for Jesus' body.

Of which council was Joseph of Arimathea a member?
It's commonly thought to be the (Great) Sanhedrin, Luke certainly implies it to be the same Council that sat in judgment on Jesus:


  50 Now there was a man named Joseph, a member of the Council, a good and upright man, 51 who had not consented to their decision and action. He came from the Judean town of Arimathea, and he himself was waiting for the kingdom of God. - Luke 23:50-51 NIV


He earlier describes it as "...the council of the elders of the people, both the chief priests and the teachers of the law..." (Luke 22:66)