WordPress has a long list of plugins os displaying Most commented posts in wordpress is very easy to do with a pluin called Most Commented which displays a list of most commented posts in your sidebar.

First You need to download and activate the plugin. Then you need to place the following code in your sidebar.php or any other theme file.

<?php mdv_most_commented(); ?>

To customize the list, you can use parameters like:

<?php mdv_most_commented(10, ”, ‘<br />’, true, 30, true) ?>

Parameters You can use:
$no_posts – sets the number of posts to display
$before – text to be displayed before the link to the post
$after – text to be displayed after the link to the post
$show_pass_post – whether or not to display password protected posts
$duration – displays comments for recent number of days
$echo – whether or not to echo the output or return an array of values

That’s it, now you can use most commented posts in sidebar.