Well as an old school player (haven't played much of the newer stuff) I'd like to point out that.
Nefarian : for the first time meeting him and there being no tactics out there, it was (WTF is with the zerg fest!), after a few days of wipe, it became alright now Nefarian lands, there are 10 mobs running around and 10 people left alive! HOW THE HELL..Finaly trying to aoe the skeletons without invunrability pot had 7 mages and 3 warlocks dead every time! worse of all is a tank failing at stance dancing
HuHuran: Frustrating until we eventually found the best choice of poison soakers (locks were much better at soaking poisons while SL spec).
C'thun: Very interesting fight all around, thumbs up for the challange.
Gothik 60: Frustrating, it seems when the living side does well the dead side fail and force a wipe, and vise versa :/
Patchwerk 60: Wasn't hard but it was so damn frustrating to never ever have him on farm even after killing KT *DONG* OT died..DAMN!
4H men 60: it is so sad how much they nerfed these guys in Wotlk, from one of the most organised fights ever in this game starting from the pull (you pull wrong and it's a wipe) to the amount of dmg the marks made, to how long the fight took (over 10 minutes AFTER they were on farm), only true annoying part was the 8 tank requirment. I missed moving in synch with my group, and every group has a completely different sync then the other, tanks have their own sync lets not forget the invisible voids and the tanks taunts getting resisted by the lady just so that she ends up placing a void in the ONLY safe spot and cause a wipe.
As for hardest....
I'd probably vote for 4 horsemen 60
in addition, it seems like Nezoia only read about 4 horsemen, as yes the 8 tanks requirement was an annoying thing but that WASN'T what made guilds wipe at them for so long