For me it must be the Reliquary of Souls that thing was and still is a nightmare to kill.
oh let's see there are so many fights that suck for one reason or another
Those fights with multiple adds back in classic were something. I remember waiting at Garr's so everyone knew who tanked what and which warlock would banish which add (back when MC was new). Sometimes this "preptalk" lasted longer then the actual fight.
Veal was a fun fight but seeing as it effectively killed two of my raidgroups and everytime we made progress some portion of the raid was demotivated and quit hte raid so someone new had to learn it all over again ... *sigh* that was extremely annoying
chess event in Kara, felt so useless. Was fun the first couple of times and just plain boring afterwards.
Huhuran, as already said felt stupid to unequip your T2/T2,5 Items for some Lvl 50 blues with only natureresist on it. Especially as a DD who was told 'mind you this is a dps race' and at the same time was told 'wear your natureresist'.
Hydros ... well I had no problem as a ranged DD, but with my feraldruid it sucked having to carry cold resist, nature resist, normal tankequipment and catgear (on top of fireresistgear for Illidan later on), The bag- and bankspace is limited you know ...
Trial of the (Grand) Crusader - I did not like the whole instance. I see that there were some nice ideas (Twin Valkyr was a nice fight, as was anub), but the whole setting felt preposterous: Tyrion raises an army to fight the Lich King, and to see who is worthy to join he sets deathmatches in an arena against some monsters. Those who survive may fight for him. Really .. what about those who die? - I guess they will bolster the ranks of that Arthas guy who commands the (un)dead. Just stupid.
Spine (normal or heroic) as a DD it is just f***ing boring, as a healer ok you have something to do, but as a DD you are really only needed for 18 second every 90 seconds or so.
Madness (normal or heroic) nice idea, nice mechanics, but it is 4x the same fight, there is not really much difference between the first 4 platforms (mechanic wise). And it takes 12 Minutes or so to do those platforms, that is just too much of the same thing over and over again.
Was nice when it was new but is boring and feels unnecessary now.
Razorscale, but that is only because I had to do the "Kill X Dwarfs with her fire" achievement once too often. Apart from that the fight is okayish.
high Priestess Arlokk, someone always banged that gong while only half the raid was ready (or even in the room ...), also I remember having a ZA group that struggled to kill the panthers or keep the marked players alive so that boss somehow always was a gamble, even when the rest of ZA was on farm.
Al'Akir, love the fight, hate how bad people are at it though, makes it such a horrible fight!
Most hated by far was 4 Horsemen from original Naxx. I had to power level a Warrior to 60, gear him, and sit my hunter just to get our server first.
From the list definitely Faction Champions.
Out the list, it's old Onyxia and Baleroc (god i hate that fight).
Spine and Madness are both very stupid long drawn out fights. I didn't even raid during progression on them and I can't stand doing them.
Hi Sephurik