How can I get a default value in a dropdown using form API?
For required items a -select- item is added automatically. If the form field is not a required one, you'll have to add it manually, for example with the following code.

  $options = array('_none' =&gt; '-select item-') + $options; //Places an extra option at the start


If it's the first item it should be selected automatically but if you want to be safe. 

  // Use the array key, not the value.
  '#default_value' =&gt; '_none', 


You'll have to filter it out the '_none' value in the submit handler afterwards.