TL;DR Which boss required the most gear?
OP:
Was just thinking if there was any boss that couldn't be crushed by top-notch players without a few weeks reset worth of gear. Given that they generally do so even on the original release, I'm guessing not.
It's a tough question to answer because the first boss in every expansion is "harder" because people have to level first and have no gear. That's why I didn't make a poll, depending on how you count things (like class balance being totally different at the end of an expansion than the beginning) the question can be answered fifty different ways. Maybe I should have said, "Which boss would last longest until they were killed by 250 US or EU guilds?"
With anachronistic knowledge of what classes will be OP in 2.4, I don't see Black Temple or Sunwell posing any real challenge.
Obsidian Sanctum (Sartharion) + 3 is actually interesting. I believe DKs were super overpowered at launch and if the toned-down versions that existed at the end of Wrath are originally in the game, tank survival could be a problem.
Yogg 0 Light and / or Algalon...those fights I'm sure the numbers mattered, but I think a lot of it was learning the mechanics. I think a group of players that had done it back in the day could handle them pretty fast. Although undergeared Mimiron Firefighter is probably no pushover.
The Lich King fight...the tanks kinda needed their tier bonuses, right? But again I don't see it really being the hardest.
Moving to Cata, I don't really know anything about Sinestra.
Hard-Mode Ragnaros is one that comes to mind. IIRC that fight was quite involved, but I think a raid geared out in Blackwing Descent / Throne of the Four Winds / Bastion of Twilight could probably handle it.
Dragon Soul Meh.
Pandaria I didn't raid hardcore, but I don't recall the first tier being that hard. I had heard Ra-den was challenging in Throne of Thunder. And Siege of Orgrimmar had some fights that were the heyday of boomkins and hunters.