This story came to my mind a week ago when my guild repeatedly wiped to heroic Sha of Pride in the Siege of Orgrimmar ... A boss which we technically have on farm. After the fourth or fifth attempt in which we died to the enrage timer, I leaned back in my chair and thought, "This is it. This would be my punishment in hell."
After thinking on it further I decided that while heroic Sha of Pride farm wipes are bad, there is likely another boss that I would find even more unbearable to repeatedly wipe to. Some of the long fights like Garrosh or Ragnaros 2.0 would be contenders. Then there's the fights with annoying mechanics like Razorscale or Faction champs. Then there are those two encounters at the end of the raid-instance-which-must-not-be-named ... Anyway, I still haven't decided what the ultimate punishment for myself would be, so I thought I'd ask you all for your own. Let us know which boss and why!