Master Chief Petty Officer of Starfleet
From Starbase 900 Wiki
There exists one post, Master Chief Petty Officer of Starfleet, or MCPOS (sometimes pronounced 'MIK-POS') which is unique. The holder of this post is the most senior enlisted member in Starfleet. His or her exact duties vary though he generally devotes much of his time to traveling throughout Starfleet observing training and talking to members and their families. While the MCPOS is a non-commissioned officer, this billet is protocol equivalent to a Vice Admiral.