The fav feature is the like button. You can see your most popular tweets at http://favstar.fm/users/<yourtwittername>;

From there click on the recent button to see what tweets of yours were recently faved, or the given button to see what you have faved.

I've recently started integrating RT tracking too, as tech tweeters do tend to use RT more than likes.

If you log into http://favstar.fm once, it will start collecting favs and RTs you give and receive almost instantly. Otherwise it collects them more slowly using twitter's REST api.