hi we already have two dogs one of each sex
the female is the oldest and boss (2years) followed by the male (1 year)
we are going to get another dog and wondered which sex should we consider
both dogs have been "done"

A lot will depend on breed, as some are more dog dominant than others and sometimes smae sex do not get on, or males do not get on.  If this is not an issue in the breed, then have whichever you fancy, as it will assume bottomof the pack at least until fully mature if there is a good age gap.