Old School Nefarion in BWL.
Part of it was some guild drama and a split that was a part of our progression. We were a casual guild and a big chunk of the more committed raiders moved on and formed their own guild so we had to retool and continue on.
That said, the build up in UBRS, the story about Lord Victor Nefarious and all that. "Let the games begin!" and dragons come pouring out of the doors....and then Nef turns into a big ass dragon, yells "BURN!" and I about crap my pants.
The fight was interesting because of the class calls, the adds, etc....Certainly, fights have grown more complicated as time has gone on but at the time, it was unique. And doing it with 40 people made it epic.
your hat may be nice, but I have the little white tank top that says Legendary right across my boobs. I win. (or more correctly, H wins)