BSG...I should add as a correction to the previous posts
that the ranks of 'Petty Officer' cease to be used in
series 4 and the final series, being apparently replaced
by 'Specialist' (grades 1 to 3). This is indicated in
some of the end credits with certain crew members being
given the ranks 'Sp2' and 'Sp3'. Since Dualla's promotion/
commission
(Dualla's rank was specifically given as 'Petty Officer 2nd
Class') to Junior Lieutenant,
different grades of PO are not mentioned and non-marines/
enlisted crew members are either referred to as
'Chief','Specialist' or simply 'Knuckledragger'
e.g. Tyrol before and after his demotion,resignation
and re-instatement as (the) Senior Chief.

There is no mention of any 'Master Chief Petty Officer',
although it has been indicated that Socrata Thrace was
a Sergeant-Major in the Colonial Marine Corps (in-show she
is described as being a Corporal before being discharged)
so the rank of Senior Chief could be equivalent to
Sergeant-Major (i.e. Senior NCO). Assuming again that
the enlisted crew, i.e. Deckhands and Marines, of
Galactica are all in the same chain-of-command
an estimate of the enlisted ranks can be given below:

Senior NCO=Sergeant-Major/Senior Chief
Gunnery Sergeant/Chief Petty Officer
First Sergeant/Petty Officer (2nd Class)
Sergeant/Petty Officer (3rd Class)
Corporal/Specialist (2nd Class)
Lance Corporal/Specialist (2nd Class)
Private First Class/Specialist (2nd Class)
Private/Crewman (or Deckhand Recruit?)

Note: all the enlisted ranks for the CMC are either
given in show or implied by character dialogue.
Combining PO with Sp allows for consistency.