How can I change the order of the list of countries provided by the Countries module?
It's a shame Sumeet didn't continue his answer, as he got so close, but here is the alter hook:

function HOOK_countries_alter(&amp;$countries) {
  $US = $countries['US'];
  unset($countries['US']);
  array_unshift($countries, $US);
} 


It copies US, removes it from the list, and attaches the copy back to the top.

You don't say in what context you use the module. I verified that it does what it should on a country-field.