MVF is an interesting app for Twitter users.
You can connect your Twitter account to this service and it would check all the accounts following you and show you the one with the maximum number of followers.
The service would then provide you with an option to get the top five if you are willing to tweet about the service.
In my case, the most valuable follower came out to be Barack Obama. All the top five accounts were popular accounts which usually automatically follow accounts following them.
Check out yours!