wp query for popular post with simple field

The correct query is:

$args1 = array(
    'showposts'           => 5,
    'ignore_sticky_posts' => 1,
     //ORDER ARGUMENTS
    'meta_key'            => 'post_views_count',
    'orderby'             => 'meta_key_num',
    'order'               => 'DESC',
    //META QUERY
    'meta_query' => array(
        array(
            'key'     => '_simple_fields_fieldGroupID_2_fieldID_1_numInSet_0',
            'compare' =>'NOT EXISTS' ),
    ),
);
$popularposts = new WP_Query($args1);

The order aguments are always outside meta_query, see the codex.