I just did a favourite films list for the first time in years a few months back. I mostly just rank them by going with my gut for better or worse, not having a required amount of films for it and not taking it too seriously since I can change it whenever. Helps that I also keep a big, big list of every 8/10 and up I've seen just as a resource.
Yea I agree that's what I do when making these favorite lists from each movie decades, go with what your gut tells yea, which movies belongs on the list and which ones don't belong on there and consider it more of a honorable mention instead.
My 30 Favorite 80's Movies

My 30 Favorite 90's Movies