For me, it was always The Caverns Below. I always play expansions through. I braved through Pirate Warrior, fought every Highlander Priest, and faced every Odd Paladin. All those top tier decks that beat me, I still played. Un'Goro META was the only time I stopped completely. Once you saw that Quest, you knew the game was already over.
I guess runner-up for would be Conjurer's Calling.