1. Dragon Soul, just didn't feel like a final raid to me. The recycled zones and encounters made it feel thrown together. I think because it was hard to give Deathwing a personality beyond the giant evil dragon cliche. Least favorite since I felt it wasted the opportunity to be something epic and original.
2. Trial of the Crusader, again felt like a cop out. Could have had more potential but was simply to direct with each boss subsequent after the other.
3. Gruul's Lair, just because... I can't think of a third.