For me it is, hands down, Quest Druid.
The inclusion of Kun put a ridiculous deck just completely over the top. I hope to never face it again.
I imagine a lot of folks are going to say that Evolve Shaman was their bane, but I didn't find it to be too terribly difficult to play against. I must admit, I played this one a lot myself and rather enjoyed it. It did not feel as bonkers to me as Quest Druid, though.
So... what "cancer" are you glad is now gone from the meta (at least for now)?