What is the highest nco rank in the marine corps?

Sergeant Major is E9 which is the highest NCO "pay grade" in the marines. Command Sergeant Major is E9 as well but has more seniority than just a Sergeant Major. Sergeant Major of the Marine Corps is also E9 but has the most seniority of any NCO in the marines. E9 means that you are enlisted and you are rank 9. E9 is used to represent the pay grade.