ToC was arguably the worse raid (and worst raid in WoW, in my opinion), but I was more disappointed with Dragon Soul.
I was expecting ToC to be a filler raid - merely gearing towards the final confrontation with the Lich King. It was disappointing despite low expectations. Dragon Soul on the other hand, was an end of expansion raid. Based on previous experience, I was expecting something much more epic - to really showcase the battle against Deathwing to preserve Azeroth itself.
With much higher expectations, the disappointment was much more acute.