Master chief petty officer
From Halopedia, the Halo wiki
Master Chief Petty Officer is the highest enlisted rank in the UNSC Navy. According to modern military ranks it is above Senior Chief Petty Officer. It is equivalent to Sergeant Major in the Marines.
File:150px-Cheif Again.jpg
only known person in halo to hold the rang
Known Master Chief Petty Officers
[Spartan-O666]