For me it was Kael´thas Sunstrider with T5 gear, we wiped so god damn long on him. And we said, when we cant get him down we havent earned us the right to go BT or Hyjal, it was the time when te pre quests were taken away. After Kael´thas it was Archimonde... "I pushed the god damn Elune thing but it was not active" i heared that so so so often. In WotlK i stopped raiding cause everything was so damn easy in naxx
(Sry for the bad English)