Master chief petty officer

Master Chief Petty Officer (MCPO) is an enlisted naval rate, a type of military rank used by the service organizations of different cultures. As a traditional naval grade, MCPO is the most senior grade of Chief Petty Officer, and is almost the highest noncommissioned officer grade in organizations like Starfleet and the US Navy. In comparison to infantry ranking systems, this rank is approximately equivalent to the more senior sergeant grades. Personnel of this grade can be addressed simply as chief (or more properly, "master chief"), and sometimes by their job title, such as "master chief specialist."

From Memory Alpha -- the free Star Trek reference

In addition, there exist two other grades of Master Chief: Command Master Chief Petty Officer (CMCPO) and Master Chief Petty Officer of Starfleet (MCPOSF). The former is the senior enlisted man in a Starfleet command structure, whereas the latter is the senior enlisted man in all of Starfleet.