The harder ones that I know of are probably Arrested Development (easier with two healers or two tanks), Headed South and probably Pardon Denied.
The hardest part of any of them is trying to do them in PUGs .
Just one shot Arrested Development (used 2 healers) and Pardon Denied without any problems. A lot of the difficulty level for each achievement has to do with which classes you happen to have in group.