Official comment
Thanks for your feature request!
While it is being considered, there is a system field that can help in getting the actively most engaged contacts at the top of the Contacts Collection.
Add the "Last Activity" field to the collection view, and click on it to sort. This field updates with the date and time that any contact interacts with your marketing efforts: opening an email, clicks a link in an email, visits a landing page, fills out a form, etc.
This field is used in our Active Reengagement Campaign and can be added to other campaigns to filter out Contacts who are any number of days past their Last Activity date (such as "90 Days Past Last Activity").
Another option is to use Lead Scoring rules to set up any kind of criteria you want; this is ideal when you don't care too much if they visit a landing page that would update the Last Activity field. Lead Scoring works retroactively too; if you add a Rule giving them 100 points for opening an email and degrade it by 2% per day, the system will apply the Score and degrade it for past email opens.
We'll keep your feature request open, of course, but I did want to make sure these other options are known!
