Lists Top Authors by Most Recent Posts

Do a WP_User_Query and then go through the results:

$wp_user_search = new WP_User_Query( array( 
    'role' => 'author', 
    'fields' => 'all_with_meta' 
) );
$editors = $wp_user_search->get_results();
echo '<pre>';
print_r( $editors );
echo '</pre>';