Which World of Warcraft boss was the hardest to kill when it was current content?

World of Warcraft Classic, if nothing else, is a very interesting look at the difference between both boss design and player approach to said design in former and current content. Blackwing Lair released this week in WoW Classic and was subsequently cleared in its entirety in less than an hour. Back in Vanilla WoW, any guild pulling the same stunt would have been accused (rightfully so) of hacking the game. I’d honestly be surprised if Blizzard would have hypothetically fallen on the side of the guild in that sort of situation. It was just unheard of to be able to clear content that quickly.

And yet, players have gotten better, resources have become more abundant, and content is being cleared quite quickly (especially when compared to the old days). But it does make one wonder — at what point does it stop being an issue of player skill and comprehension and turn into an issue of the boss being really dang hard?

For instance, let’s say we hypothetically had servers for every major raid-release patch available at our disposal. Which of those servers/raids would take the longest to clear? Would it be Sunwell Plateau, with its notorious raid-stacking requirements? Would it be Ulduar, with its nearly impossible Yogg+0 encounter? Would Cataclysm’s Throne of the Four Winds continue to be a huge hurdle for raids to clear in a timely fashion? Or would we find that, in the end, no content is harder to clear than the present content?

Personally, my vote goes to Kil’jaeden in the Tomb of Sargeras being the hardest encounter to beat at-level. That’s both because it was recent enough that there’s little room to argue against it being hard mechanically and because it was hard enough in its own right to cause a crazy number of wipes, even for top guilds. But then again, I could be giving it too much credit. What do you guys think has been the hardest encounter to clear at-level? If we were given every resource available to us at the time of original release, which boss in World of Warcraft’s long history would take us the longest to kill?