If you get really stuck with this, contact me, and I will build the widget for you in exchange for you linking to my blog.
You can see my effort at this, there it is, at the top left hand sidebar. It displays the top ten polyborers (commentators) in descending order and links them to their last comment.
To do this you are going to need a yahoo ID so that you can log into yahoo pipes and you will need to register with widgetbox.
My polyborers widget is displaying a stripped down rss feed. Thats where yahoo pipes comes in, it strips out just the names of those who have posted a comment, adds up the number of entries and displays them in descending order. You are going to need to set up your own pipe but don't worry this is very straight forward because you can use my polybore pipe template.
I've done a video tutorial of how to copy (clone) the pipe and set it up for your blog which you can see here. (java) or better quality flash version.
It shows how you can block comments from yourself, anonymous or anyone else. Also you can change the truncate value if you wanted say just a top 5 or a whopping top 100. The end of the video shows how to get the rss feed from your pipe.
If it is going too quickly hit the pause button. The url you need to enter (for blogger) is "http://yourblogname.blogspot.com/feeds/comments/default"
Once you have the rss feed address of your yahoo pipe copy it to note pad you will need it for widgetbox. At widget box click make a widget. Choose blog/feed and it will prompt for URL, copy and paste in your yahoo pipe feed. http://pipes.yahoo.com/pipes/pipe.run?_id=a4a615ae04f94ebf98cf79d14ce83e5d&_render=rss) is roughly what your yahoo feed URL should look like.
After that it is just a question of configuring what the widget will look like and the widgetbox interface makes this very easy to do.
Thanks to assessmyblog whose pipe I cloned then modified with a truncate function and whose instructions got me started assess my blog. Need more help or want me to build the widget for you? Leave a comment or contact and I'll get back to you asap.