#PHP <s>135</s> 146 Bytes

I know I'm late but I wanted to post anyway:

    function a($b){
      $b=strtoupper($b);
      foreach(range('A','Z') as $c){
       $d[$c]=substr_count($b, $c);
      }  
      arsort($d);
      return implode(array_keys($d));
    }
    echo a($z); //let $z be the supplied string.

golfed:

    function a($b){$b=strtoupper($b);foreach(range('A','Z') as $c){$d[$c]=substr_count($b, $c);}arsort($d);return implode(array_keys($d));}echo a($z);