How can I make one of the field's drop-down values to be a default so it's set to that field if no manual selection made?
Fields of type drop-down do not offer a default value. You can see that if you edit your field on either workspace or project level.
By default a 'No-Entry' selection is shown. This will require a user to manually select a value each time for that field.
You can use Work Item Templates to set default values to your artifact fields. That includes custom fields and also drop-downs.
You can learn more about Templates here: Work Item Templates help page