If you are looking for a site that will list the top sellers for the week/month/year, you won't find it. Studios are very tight-lipped about the sales of their video product. Sites like Amazon will list THEIR top sellers, but they won't show the overall sells from every store/site.
The Top 100 DVD list that I put together was based on an extensive polling of several of the Internet's most popular DVD discussion boards.
The results site is: http://www.angelfire.com/dc/top100dvd/