Best way to make new groups mailinglists by default
I would create a custom extension as explained here or using an existing one and implementing hook_civicrm_buildForm

Updated

function myextension_civicrm_buildForm( $formName, &amp;$form ) {
  if ("CRM_Group_Form_Edit" == $formName) {
    if ($form-&gt;getAction() == CRM_Core_Action::ADD) {
      $defaults = array ("group_type[2]" =&gt; true);
      $form-&gt;setDefaults($defaults);
    }
  }
}


That will make the Mailing list checkbox checked by default.