But after thinking about it, I had to go back to Burning Crusade, the Black Temple, and Illidan. The storyline required for even getting into the Temple in the first place was amazing all on its own, but the raid instance was the pinnacle of Burning Crusade, for me -- and Illidan was the icing on the cake. I'm kind of sad that he can be soloed so easily these days, and players don't even see all of the phases of the encounter anymore. Having a cloth-wearing warlock tank shadowy Illidan was ridiculously cool. And even if I didn't get the warglaives until after the expansion's end, it was still a satisfying encounter that just hit all the right buttons for me.
There are others, of course -- Kil'jaeden, Yogg Saron, Mimiron, the Lich King, and many more. And I haven't even touched on dungeon bosses at all. Vanessa VanCleef was one of my favorite dungeon fights to come out of Cataclysm. But for now, I'll leave my answer as Illidan. What about you guys? Which boss, in all of the dungeons, raids, I'll even take scenarios if you'd like, is your favorite? Which one is still fun to beat? Which one was so ridiculously cool you could hardly stand it?