Matt Rossi: If we're talking "hardest for people in the best gear available to do at the time", then C'thun. They HAD to nerf C'thun before anyone (and I mean anyone) could kill him without exploiting. For a group of level 60's in BWL/AQ gear, pre-Naxx, C'thun was the hardest fight. Not even Naxx fights compared. There has never been a fight tuned that high again.
Joe Perez: I agree with Rossi, C'thun was by far the hardest fight. You couldn't even pull the boss without losing people immediately. That fight was so terrible I remember my guild at the time having to take a break from raiding for a week as a group because it was so damn stressful. It remains probably the only fight that has caused me to actually visit harm upon a wall near me while raiding.
2. M'uru (pre nerf)
Allison Robert: I can't have been the only tank who woke up in the middle of a nightmare screaming, "Shadowsword Berserker has gained Flurry! Shadowsword Berserker has gained Flurry!"
Matt "Matticus" Low: Pre-nerf M'uru takes my vote.
3. Heroic Lich King 25-man at 0% buff
Dawn Moore: Heroic Lich King 0% buff. It was killable, but no one killed it before 5% came out. I started attempts at 10% in my guild and it was a ridiculously unforgiving fight then. Not quite instant death at pull, but definitely death within the first thirty seconds to a minute due to any number of issues. I can only imagine how fantastic 0% was.
I should point out that it's pretty normal for any guild, when pulling a boss for the first time, to die to any number of things. It takes time for people to get a hold on everything that's going on. For that period of time to last for more than an hour or two is very uncommon, especially in a guild raiding 20-25 hours a week. We spent all of 10% and most of 15% in phase 1. I didn't really learn phase 2, or even see phase 3 until 20%.
4. Yogg-Saron +0 25-man
Allison Robert: Absolute brutality. Yogg could, and often did, RNG even the best players into a sliver of sanity approaching phase 3, and there was nothing you could do about it. The margin for error on this fight was close to nonexistent.
Scott Andrews: Besides C'Thun's original bugged version (which was actually impossible), my vote is for Yogg +0. The first guild to beat it did so using an exploit. When they got caught, they claimed that the fight could not be beaten as designed. That turned out to be untrue, but it was still punishingly difficult, as intended.
Runners up: Alagalon, pre-nerf heroic Mimiron, and TK Kael'thas.
Anyway, back to our top four up there. I tend to think that's a good set of candidates for consideration for WoW's hardest boss. It's true that C'Thun was initially unkillable, as Rossi pointed out. However, that was at least partially due to a bug that existed at implementation, where C'Thun's tentacles in phase 2 would spawn in places they weren't supposed to (such as inside C'Thun's stomach) and weren't reachable by DPS, not only because he was just that tightly tuned. Or, to put it a different way, C'Thun was that tightly tuned... by accident. Once the hotfix that ended the tentacle-spawing problem was deployed, C'Thun went down. Now, that doesn't mean it wasn't a brutally punishing encounter. C'Thun deserves the reputation he has, and, as Joe mentioned, you couldn't even pull the boss without losing some raiders, and that design was intentional.
I'm surprised that, though M'uru's name came up immediately in my inquiry thread, no one went into detail as to why. I have vivid memories of angry of forum posts, impassioned discussion, and guild laments during his time as progression content. As the second-to-last boss of Sunwell Plateau, M'uru took the cake in an instance brimming with encounters that left raiders in misery. In fact, though the quote is lost to the old WoW forums that disappeared with the Blizzard website revamp, I distinctly remember a developer stating flat out that they would likely never design anything as hard as Sunwell ever again. Thank goodness for that.
M'uru's difficulty could really be pinned to the hybrid nature of the encounter (both a DPS race and an add-control fight) and the fact that both M'uru and his five different types of adds had a lot of health and hit really, really hard. Once M'uru transformed into Entropius, in phase 2, it became an all-out DPS race as the multi-target damage he dealt ramped up every 12 seconds to eventually overwhelm the raid. Very unpleasant.
Yogg +0, meanwhile, simply took what was already a difficult encounter and then removed your tools for coping with it. Without Freya, your healing is weakened and you have no way to regain lost sanity. Without Mimiron, you're slower and the boss is faster. Without Hodir, you take more damage. Without Thorim, you have less health and the Immortal Guardians are, truly, unkillable. Yogg-Saron has an enormous number of abilities, as well as multiple adds and three phases. It's a complicated and overwhelming fight with a lot of moving parts. Without the aid of the Keepers of Ulduar it became next to impossible. There were servers that took more than a year to get a Yogg +0 kill. Ouch.
In the end, I think the honors probably go to C'Thun, partially thanks to one aspect of his fight that is unique to our top four: it required 40 people. Coordinating 40 people to accomplish that kind of task is an achievement in and of itself, and no other boss we picked includes that same combination of high damage, multiple adds, and sheer management. Really though, in all honesty any one of these four bosses could easily be considered the hardest that WoW has had to offer.
What do you think, readers? Which one of the four would you pick, or would you choose something else entirely? Tell us about it in the comments; we'd love to hear your thoughts. And good luck on all the new challenging bosses that are sure to wipe your raid in patch 5.4!