Is it possible to get object which has the highest number of related objects?

I want to choose most used plan which is a plan with the maximal number of UserPlan objects.

I can get the number but not the instance.

number_of_users = Plan.objects.aggregate(max_users=Max('userplan'))['max_users']

code:

class UserPlan(Model):
   plan = ForeignKey('Plan'..)

class Plan(Model):
   ...

    @staticmethod
    def favorite(self):
        number_of_users = Plan.objects.aggregate(max_users=Max('userplan'))['max_users']
        # ?

I could find it using loop but it could be slow.