I would avoid forwarding the email itself because people receiving the forwarded email would be cookied as the original recipient, artificially inflating activities, scores and so on. Furthermore they would not be
marketable.
The forward to a friend is a safer alternative. That is displayed in the leads' activity log:

The token is {{system.forwardToFriendLink:default=}}
The activity is also displayed on the recipeints' log

A smart list having the filter "Sent Forward to Friend email" or, more flexible, the corresponding trigger to log an Interesting Moment could be scrutinised through SOAP API' ActivityType InterestingMoment in order to identify the highest achievers.