My vote goes to HoR, only heroic I have not completed, I am going to try it with my new team but my fully geared pally tank, 2 warlocks, hunter and resto druid have never completed it. New team with Shammies might work better, I will see when they get geared enough
Other heroic which can be a pain is The Old Kingdom, poision is reallynasty in the first few pulls there and the last boss is also a pain.