Aside from overturned bosses that were all of impossible to kill due to bad design pre-nerf I think that the original iteration of the four horsemen was the hardest most complex fight in the history of World of Warcraft and I don't know if it will ever be beaten in that aspect. The encounter didn't have as many different mechanics compared to some of the modern day bosses whose raid info page is a mile long. But, the encounter had a full 40 man raid group broken up into many little groups that all had to rotate correctly or pretty much be killed instantly.
In a raid group unless you're in Paragon there's typically at least one tard that the rest of the raid is covering for and that's always made very apparent on boss encounters where the raid has to split up and do multiple things at the same time. This has never nor will it ever again (unless blizzard brings back serious 40 man raiding again) been more apparent than on the original four horsemen.
There were 4 tanks all tanking separate mobs and they had to rotate based on stacks of the debuff and with taunt being spell hit back then there could be some hairy situations due to taunt resists.
You had between 10-12 healers who all had different roles some following tanks around but because of the tanks higher health pools the healers had their own separate rotations they had to follow.
Then you had 24-26 dps split up into different groups. There was no zerg the meteor boss strat back then because the dps just wasn't high enough so you had to rotate out in multiple groups. You had some groups resetting stacks in the safe zone, you had some on Morgraine and then you always had to have some with the tank to soak meteors. You also had to run in and out together because other wise you risked taking a stray meteor to the face and being one shot.
People also had to deal with invisible void zones which some people did use that as an excuse when they just tarded it up it was a buggy mechanic that really did exist and would one shot a healer if they didn't get out before the damage from it ticked (think of moving on heroic lich king except sometimes defile was invisible and instantly killed you when it ticked it's first damage). Not so much of a mechanic but a common bug that had to be dealt with.
But, by far the hardest mechanic was getting 40 people to split off and become extremely fragmented and yet still function as a whole and to do their jobs with it being almost impossible for the raid leader to see the entirety of the battle field and keep an eye on everyone even if someone could keep track of that many people moving at different times.
So, I'd propose that the original version of the four horsemen was the hardest boss encounter of all time and will never be beaten just based solely on the amount of coordination it took to down that boss and the fact that there aren't 40 man raid dungeons anymore.