In Cataclysm, a number of different raids and dungeons were added to the game, some with mechanics never before seen in the World of Warcraft. Before this, there were numerous encounters that tested raiders' ingenuity and button-mashing skills to the limit, causing players to think up new and questionable (Saronite Bombs, anyone?) ways to counter them.
What has been your favorite raid, instance or specific encounter? Mine would have to be a dead tie between Molten Core and Ulduar; the epicness of watching my brother being the only one left standing during a Ragnaros fight, compared to the dark whispers of Yogg-Saron as you rip your hair out of your skull, trying to get that last 10% down without being overwhelmed by invincible adds. A lot of these fights have been epic; a lot, a pain. But all of them have meant something special to those who have beaten them.
What has been your favorite fight, raid, instance, or outdoor raid, and why? Was it because of the people you were with, the joy of downing a hard boss after the frustration of wiping, or the wonderful purple (or blue) loot that flowed in afterwards?