Nader

The “Most Comments” plugin displays as the name says posts with the most comments. You can either use the widget in the sidebar or integrate the plugin as template tag in your theme files.
Features:
<ul>
<li>Limit the amount of posts to be shown</li>
<li>Show comments from a specific post category</li>
<li>Exlude posts without comments</li>
<li>Select a time range (last XX days) of posts to be taken into account</li>
<li>Customize the design template via the settings page</li>
<li>Place a widget in your sidebar</li>
</ul>
More Information & Feature Description: 
http://www.brandinfection.com/wordpress-plugins/most-comments/
<h3>Configuration</h3>
You can configure the most comments plugin to your needs. Options can be found under the menu <code>Settings > Most Comments</code>

Most Comments

Display posts with the most comments, as a widget or on a page using the template tag. Customize the look with specific settings.

Robin W

This Plugin lets you add ranking elements to topics, replies and profiles in bbPress
These appear after the author details on each post and can show any or all of the following in ANY order
Total Topics eg ‘Topics : 78’ – you can set the wording and syntax 
Total Replies eg ‘Replies : 157’ – you can set the wording and syntax 
Total Count eg ‘Total posts : 135’ – you can set the wording and syntax 
Your name for that level – eg ‘ace contributer’ 
Some stars for that level – in the colour of your choice 
An image for that level – you can specify the size.
You can also style and add badges to users, such as ‘Key contributor’

bbp user ranking

This is awesome. Just what I was looking for. Thanks Robin.

Just what I was looking for

For bbPress - Lets you add ranking and badges to topics, replies, and profiles

Most Comments vs bbp user ranking