Also, lets not forget one thing. There are FAR more mods and addons now that simplify how message is conveyed to people and help with their rotations. The only real help people had in Vanilla was deadly boss mods, and those notifications that it telegraphed are now build directly into the WoW client.
Vanilla level 1 raids on Hogger.
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I would argue the opposite for the same reasons you listed here.
Vanilla Naxx was the most difficult - You needed several sets of gear, you needed 40 people who had those sets, and the pool of addon's to use was slim to none, and the general raiding experience of the players was no where near what it is today. For instance when you get a "Patchwork" fight - an encounter that boils down to a DPS race/check on the boss. Well, Patchwork was a boss in Naxx where this term comes from. Additionally to that, Naxx was on of the least accessible raids in WoW history with I think only 5% of the player base ever stepping inside. That was the reason they started to alter things to make the raids more accessible to get into.
I think that question get's a different answer depending on the role you played.
For example as a healer in MC you would hardly see the boss and know less about the tactics involved, damn crossheal.
I don't think mechanics these days matter less at all. My guild is working on Mythic Gorefiend at the moment and everything about that fight is like that. People not placing doom at the very edge of the room? Wipe. People's spirits getting killed too quickly? Wipe. People's spirits not dieing quickly enough? Wipe. Tank getting broken out before his add gets out? Wipe. Any little thing going wrong is a very quick and obvious wipe, we've had loads of sub 2 min attempts.
I did C'thun back in the day too when it was bugged and unkillable, the eye lasers on entering the room was not an issue after a couple of attempts to get it down. We would get through the eye phase easily every time, but phase 2 always overwhelmed us really quickly until the bug fixes/nerfs.
Barring bugs and things that were fixed later, WoD is the most difficult raiding period. The movement, mechanics and the like are much more difficult than the old style encounters. If you put the current mechanics back into vanilla some of them wouldn't have been killed for a VERY, VERY long time, if ever with things as they stood then.
"Old WoW was when the game was bad yet good. New WoW is when the game is good yet bad." -Destil
The next xpac will always be harder than the previous one. Raiders are always getting better (even if it doesnt appear to be so). So if you mean which XPAC was hardest at the current time, I couldn't say, but difficulty in hindsight? Wod>MOP>CATA>LK>BC>Van ... If you through a team of somewhat competent WOD raiders into BC/Van, it would be a shit show of destruction, even though at there time, the content was hard. Note, I am going off pure BOSS difficulty which is really all that matters.
Vanilla and TBC, but only because of the rooster boss and how buggy everything was.
I honestly think all you'll get now will show how many people raided in Vanilla/TBC and how many people started after Cataclysm.
Most people don't have the full 10 years experience to make an accurate judgement.
Before you say "It shouldn't matter". Imagine if someone came up and started arguing the superiority of the most recent 3 Star Wars movies, when you try to bring up the original 3, the reply they never saw them. How can their opinion be relevant in any way.
WoD is the hardest, but MoP comes really close to it, especially ToT in general or SOO mythic/old heroic.
Please take into account that I didn't/haven't raided in MoP/WoD. I find the fights to have gotten harder with each expansion, when you consider the mechanics.
However, I pugged 9/12 heroic ICC, and finished T11 with 9/13 bosses. The heroic (mythic) bosses were way harder in T11 in my opinion. I'm actually happy we didn't even get to try heroic Ascendant council, because damn, what a mess!
I'm voting Cataclysm, based on my own experiences.
I did watch Method's kill video of Archimonde, and that looked hard, and I've heard people struggle on Gorefiend, so I feel mythic HFC ain't no walk in the park either
Vanilla MC raiding would have been the hardest. Im talking right at the beginning, no mods, no guides, little to no experience with boss mechanics. Having to organise 40 people and making sure they knew what they were meant to be doing and when was a nightmare. Resist gear farming was horrific.
The game was new, concepts were new and we had little idea what to expect.
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Although if you want hardest ever boss I would say AQ40 prenerf. Cthun was nigh on unkillable at first. Luckilly my guild got held up on twin emps till the nerf hit
If by head you mean getting people together and do something to get them prepared to just be able to have a chance like gettin resist gears then I think older xpacs are harder. If you mean executing mechanics properly then I think now is harder. I consider now myself pretty terrible when I was in TBC. Mechanics back then was pretty easy compared to some Mythic(Old heroic) fights like H LK,H rag,H Lei Shen,etc. The mechanics were not as well designed too.
if we talk about tactic - MOP and WOD brings most challenge boss in my vision.
But if we talk about "gather ppl that have required gear and skills" - vanilla was much harder. Flasks costs a lot. And it was really necessary. So every1 farm black lotuses hardly. Fire/Frost resist gear cos MUCH. Plus you need to be attuned. Plus 40ppl. Plus shit internet connection cos at 2005 most ppl sit on dial-up. 33.6-56.6kb/s. And lags during raid was regular. Many bosses was badly tuned and even was bugged. No youtube. So you cannot really read or watch tactic. Only personal experience. And experience of your guild. Newcomers know absolutist NOTHING. And also they need to finish prequest just to join. Plus massive grave yard runs after wipe. Now u can pull after 30sec. Massres, eat and gogo. Previously u need run like 6-7min to corpses. So most of time u spend not on boss but on farming herbs and runs from grave. 30 sec pull, mistake and 7 minutes to get get ur corpse. Thats why it was hard. Also 2-3 epics from 1 boss in 40ppl raid. Feel da pain. Now full gearing like 3-4 weeks. In Vanilla it was around 6month if you are VERY lucky and officer at same time.