Argenteuil is the second most populated town in the suburbs of Paris ( after Boulogne-Billancourt ) and the most populated commune in the Val-d ` Oise department ; however it is not the prefecture . Argenteuil is the second most populous commune in the suburbs of Paris ( after Boulogne-Billancourt ) and the most populous one in the Val-d ` Oise department , although it is not its prefecture , which is shared between the communes of Cergy and Pontoise .