I think we did Tsulong in a night? Meh.
I'd go with Zorlok if anything. Also, who voted for Blade Lord?
Feng the Accursed
Gara'jal the Spiritbinder
Reset kings (spirit kings)
Will of the emperor
Imperial Vizier Zor'lok
Blade lord Ta'yak
Wind Lord Mel'Jarak
Amber Shaper Un'Sok
Grand empress Shek'zeer
Protectors of the Endless
Sha of Fear/Gear/Naps
Up to the point I did, I found Spirit Kings the hardest if it's not outgeared. HC Amber Shaper is unforgiving as hell, but not hard per say. I have yet to see Shek'zeer, Protectors, Tsulong and Sha on HC.
Heroic Sha of Fear was far from the hardest. Not really too much to learn, only difficult part was keeping focused for 20 minutes. Also Heroic Empress was a joke with Disc Priests. My vote is Heroic Tsulong (post cheese nerfs) or Heroic Amber-shaper because people suck at hitting buttons at the right time.
Heroic Vizier took us two weeks to down because it constantly bugged on us and wouldn't reset. We had to fucking spirit rez multiple times because we'd just die instantly zoning in into song.
Not hard, but so far the most time consuming hard mode by far and the most frustrating.
i found Grand Empress to be quite hard (DK tank.)