<?php
	$args=array(
	  'cat' => 3,
	  'orderby' => 'comment_count',
	  'order' => 'DESC',
	  'post_type' => 'post',
	  'post_status' => 'publish',
	  'posts_per_page' => 6,
	  'caller_get_posts'=> 1
	);
	$my_query = null;
	$my_query = new WP_Query($args);
	if( $my_query->have_posts() ) { ?>
	    <ul>
	    <?php while ($my_query->have_posts()) : $my_query->the_post(); ?>
	    <li><a href="<?php the_permalink() ?>" rel="bookmark" title="Permanent Link to <?php the_title_attribute(); ?>"><?php the_title(); ?></a></li>
	    <?php  endwhile; ?>
    	</ul>
	<?php }
	 
	wp_reset_query(); ?>

How to Display Most Popular Posts from a Specific Category 

wordpress

PHP