$title = apply_filters('widget_title',
empty($instance['title']) ? __('Recent
Posts') : $instance['title'],
$instance, $this->id_base);

function new_widget_title($title) {
 if ($title == 'Recent Posts') $title = "Greatest posts";
    return $title;
    }

add_filter ( 'widget_title' , 'new_widget_title', 10, 3);

function widget( $args, $instance ) {
    extract($args);
    $title = apply_filters('widget_title', empty($instance['title']) ? __('Meta') : $instance['title']);
}