The "Best Of" mode uses a percentage ranking of posts.

db/migrate/20130322183614_add_percent_rank_to_posts.rb

@@ -0,0 +1,12 @@
+class AddPercentRankToPosts < ActiveRecord::Migration
+  def change
+    add_column :posts, :percent_rank, :float, default: 1.0
+
+    execute "UPDATE posts SET percent_rank = x.percent_rank
+              FROM (SELECT id, percent_rank()
+                    OVER (PARTITION BY topic_id ORDER BY SCORE DESC)
+                    FROM posts) AS x
+              WHERE x.id = posts.id"
+
+  end
+end