Heroic Spine of Deathwing, 25man: I hated this fight with a passion.
At first I hated it because I could not possibly contribute as my favorite spec affliction and call it even with the rest of my guild's raid. I could pass by on normal, blaming my lack of burst for my low-metered results, but that wasn't going to cut it on heroic.
Then I started to hate the fight as I struggled to squeeze out every last drop of damage I had in me, even min-maxing my offspec demonology to progress with. Warlock hell, they called it. What a lockblock! My anger started to extend to things outside of WoW, emotionally and physically, because I was so frustrated with my apparent failure to kick some Destroyer derriere.
But the fight really opened up a lot of the finer points of DPSing an encounter. Heroic Spine reminded me that the fight isn't all about the end DPS number when the combat logs stop flowing.