Yeah, I voted for Naruto based on its quantity, quality, and variety of fanfic and thorough exploration of the time-travel, fix-it, and SI/isekai genres. It's also a fandom where you can find epic wish-fulfilment action-adventure fic with female protagonists, which is rare. (Canon may have done all the female characters dirty, but fandom grabbed them and ran away cackling madly. I like that in a fandom.)

I like Good Omens and wish it well, but it would take a lot more intense feelings than I have for GO to get my vote over the fandom that spawned Dreaming of Sunshine (and the resulting library of DoS-based recursive fic).