I'm gonna say Sons of Flame in Ragnaros / Rag HC because IIRC, Defile required just one person to execute it properly? On Ragnaros, if you had one guy failing and his add reached the hammer, it was game over. Still remember our DK death gripping the add INTO the hammer like it was yesterday.
Other contenders are of course, Defile, Wind Bombs on Mel'jarak, a few different mechanics on Zor'lok HC, Nefarian interrupts, not stopping DPS on Spirit Kings HC, too much DPS on Maloriak, and quite a bit on Ascendant Council HC.