WordPress Popular Posts allows a shortcode to list posts or pages sorted by the number of comments that they have received.
An example of the shortcode to display the 10 most commented posts would be this, after activation of the plugin.
10 Most Commented Posts
wpp range=all stats_comments=1 order_by=comments post_type=post,page limit=10 wpp_start=
The above must be enclosed in square brackets though. 
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