Heroic Malkorok down! Definitely the hardest heroic so far in this tier. It's not numbers so much as personal responsibility and execution. There almost no room for error; usually a mistake means death and that means a wipe because you'll miss the enrage.
Heroic Kor'kron Dark Shaman down! We're over halfway done with Siege of Orgrimmar heroics now. After a totally demoralizing 1 health despawn (we have video proof) we bounced back and got them down the next night. The split group strategy makes the fight much more manageable, but increases the likelihood of despawns. We even had a slime evade bug on a tile in the middle of the room and cause a despawn. Basically, this fight is kinda bullshit because of the environment and this is why boss fights need flat floors.
Heroic General Nazgrim down! Not a particularly difficult fight, most of the difficulty comes from personal responsibility in avoiding excess damage and proper target prioritization combined with the length. The added heroic mechanics aren't particularly challenging or interesting.