Oooh Vanilla WoW... The memories.
I played until the release WotLK (which I bought and played for a week after which I stopped playing but continued paying the subscription for another year... sigh). I was a hardcore raider originally on Kargath (where we had several server firsts) and then on Thrall when Kargath became unstable. On Thrall we competed boss to boss with Ascent for server first kills in Naxxramass - the best dungeon of vanilla and burning crusade together.
The hardest fight in the first 4 years was in my opinion was Kael'Thas before the nerf - gosh the insanity of that fight... Illidan was a joke compared to it. Overall I think Burning Crusade's raiding was very imbalanced - the fights were too hard overall - progression was stymied and many generally good players and guilds just gave up at that time. I personally was the second tank in Vanilla and than the Main Tank of the guild during BC.
Other fantastic fights outside Naxxramass were K'Thun, Twin Emperors, Illidan, Illidari Council.
Naxxramass was a jewel in the crown. Every single boss fight there was exquisitely designed. No jokes no nothing there. The Four Horsemen were an exercise of perfect timing and control. Thadius required almost lag-less connections (which I agree is probably a bit too much to ask) from the whole raid. But my personal favorite is the Gothik fight - the most intense even if not the most difficult...
Foolishly you have sought your own demise. Brazenly you have disregarded powers beyond your understanding. You have fought hard to invade the realm of the harvester. Now there is only one way out - to walk the lonely path of the damned. Oh Gothik we sooo loved you!
There are of course the bitter memories of guild splits and drama - I don't want to reminisce on them so I won't.