For me, Blackwing Lair was probably the pinnacle of class WoW raiding. While Naxxramas was exhilarating and bold with its mechanics, Blackwing Lair felt more like storming the castle. The bosses had some great dialog, the scenery was impressive, and there were awesome healing trinkets that I coveted for years. Let's not forget the epic fight with Nefarion at the end where hunter's broke bows, mages polymorphed friends into grubs, and everyone was cloaked in Onyxia's scales.
Which classic WoW raid was your favorite?