How to get Most popular posts in WordPress.
Add code in your theme where ever you want the popular 5 posts to be displayed. by default show most popular post
    <?php $pc = new WP_Query('orderby=comment_count&posts_per_page=5');  // Which post show on page
      while ($pc->have_posts()) : $pc->the_post(); ?>
		<li><a href="<?php the_permalink(); ?>" title="<?php the_title(); ?>"><?php the_title(); ?></a>  //show post title
		<p>Posted by <strong><?php the_author() ?></strong> with <?php comments_popup_link('No Comments;', '1 Comment', '% Comments'); ?></p></li> //show post comment 
      <?php endwhile; ?>