How to get rid of default font (Helvetica Neue) in Numbers?
You need to update the table style so that your changes persist for new cells. Make the changes to the formatting of the table as you wish, then select the dropdown triangle for the currently selected table style and choose Redefine Style from Table (Numbers '09).

                             



In more recent versions of Numbers, simply click the blue Update button for the Table Style. The asterisk indicates changes have been made to the currently selected cell which has not been saved to the style.