Seedling's distro of Joomla includes amazing AJAX-enabled commenting system that offers a ton of administrative options but is very easy to manage.

After surveying over 20 comment systems we fell in love with Compojoom's 'iJoomlaComment' extension.

You can define which articles/posts can be commented on by category/section and/or specific articles/posts. Administering comments is also easy and can be done from a searchable table of them on the admin side or directly in the comment threads on the site's front-end.

Also included in the wealth of comment functionality; import comments from a previous Joomla installation, get notification when comments are left on your site, restrict comments - by length, bad-words, formatting and more!