Do you have a wordpress.com hosted blog?
If you have downloaded software from wordpress. org then you are posting in the wrong forum. We run on different software and in that case you need to post here http://wordpress.org/support
At wordpress. com there is Recent Comments widget that is not available in the widgets for wordpress.org installs. At wordpress.com there isn't a widget planned for Top Commentators that we volunteers are aware of.
If you have a wordpress.com hosted blog you can request one by contacting staff during support hours http://wordpress.com/contact-support/ And in the meanwhile, you could use a text widget and design it to include the linked names of your Top Commentators.