Whenever you create a dropdown field or listbox using custom values, be sure to set the default selection to the most frequently selected option.
To add a default choice, click Edit to go through the configuration of the existing Search and Report DataPage and proceed to the Configure Search Fields screen.
Select a field under the DataPage Elements panel and change the form element to Dropdown or Listbox. Select Custom values from the Source dropdown and use the green check-mark button to choose the most frequently selected option on the right of the custom values input control. The default selection will have an "X" at the far left of the display text.