I don’t know of one, but you could probably write one easily given the breadth of API wrappers out there. An example with github3.py would be
import github3
r = github3.repository('owner', 'repo_name')
most_watched = next(r.iter_forks(sort="watchers", number=1))
As best I know, you cannot sort on stars and repositories don’t have that information returned to them. You could, however, sort on forks but you would have to do it by hand.
The above example is two lines but you can achieve the same thing like so:
curl https://api.github.com/repos/owner/repo_name/forks?sort=watchers
That won’t limit how many results you get, though.