Hour of Twilight easily takes the cake. Worst raid in WoW history. Turned what could have been two raids on their own (End Time and Well of Eternity, aka Battle of the Ancients) into two terrible, terrible 5 mans. It's ONLY saving grace was Darkmoon Faire island, and I consider that more of a Cata content then a 4.3 specific content. But then that was the only content outside of the terrible 5 mans and the shitty raid.