There are lots of Spammy apps available on Facebook which shows top friends on Facebook. Those are calculated my the number of connections between both the users. But recently Arjun Sreedharan added a tiny script which will score in a Table. Its very simple.
Simply Drag the Link below and add in the Bookmark bar (available below the Address Bar).
Now login into yout Facebook account and click on the Bookmark. You should see your Friends’ Rating Score. Smaller the friend’s score, higher the rank.