Personally I am not a fan of the hard enrage
I've always kind of preferred gib enrages than kitable things.
Ulduar-style hardmode activations. E.g. kill XT's heart in time.. Whoa batman, completely different fight now begins
The only hard mode I didn't like was Hodir because if you miss by a fraction of a second you can't reset. Otherwise I liked the concept of doing things differently and making it a different fight.
Least favorite is easily any resistance fight. Whether it be as bad as Huhuran or I hated doing fights in gear completely specific for that fight that will so terrible you have no reason for it at all. Doing a fight with magic damage is one thing, making the fight so you effectively NEED resistances to do it was terrible and never should have made it to BC.