//popular post is the post who have been opened maximum number of times..
<?php
query_posts('meta_key=post_views_count&orderby=meta_value_num&order=DESC');
if (have_posts()) : while (have_posts()) :
the_post(); ?>
<li><a href="<?php
the_permalink(); ?>"><?php the_title();
?></a></li>
<?php
endwhile; endif;
wp_reset_query();
?>
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