As it happens, this was something I had partially done but not launched. You gave me the motivation to launch it, thanks!
You can now sort by Number of Favourites, and when you hover over the rating of a story it also shows the number of favourites it has:
Wait a second… zoom out a bit…
Heaven Sent is not only the highest-rated but also the most-favourited story on the site?!
I am SHOCKED I tell you 
It’s almost like it’s the best Doctor Who story ever… 