Possibly montage the top 4 which should change less often.
Although I think Interestingness is a slightly better (although sometimes weird) measure of how interesting something is. Number of faves can reflect a builders number of 11 year old contacts as much as anything ;)
Montages/collages work well for larger icons, like collection and set icons, but not really for group icons, which are the same size as our avatar icons. A cropped-down version of a single photo would probably work best.
But I agree that interestingness, not number of faves, is a better measure of what's "best" in the pool.
I'm leaning towards the screen cap of the "favorites" icon mostly because it would eliminate any competitiveness amoung the group's builders for that little spotlight. Otherwise, the most favs seems good to me. Then again, you came up with this, Aaron...why not one of yours?