All time? Pastel Horses followed by Homestuck, Avatar and Frozen.
Right now? /sug/ has lost its steam, /gfg/ is practically dead. /sveg/ are cancerous but they have mostly kept their cuck posting to themselves. SAme with the Gumball threads.
I think any autism hole is fine as long as they don't spam it. And spam it doesn't mean 5-7 threads related to it, it means at least1/3 of the board being filled with it and unrelated threads being hijacked by fans to shitpost.
So I guess that would fit the /tv/ cons--Superhero Movies wars threads as cancerous. Jesus, I can't stand this board whenever one releases.