Visiting your Contacts page you'll find all of your contacts have been assigned an 'Activity' score of 0-100. Sorting this to show the highest first is a great way to find your Most Active Contacts.
The important thing to remember about the “Most Active Contacts” is that it is all time sensitive. And the way that this feature works is it gathers your contacts that are opening, clicking, and enjoying your marketing pieces and compiles them into a list. Then, each contact is given a rating out of 100 on the activity meter. The higher the activity meter number, the more active the client.
Also make sure to check out your Contact Call List on your dashboard for a shorter list of your active contacts that may deserve some personal followup.