Best raid to me is definetly Karazhan.
It had very unique atmosphere and overall it was very fun. Since Karazhan no raid has really given that same feeling. There was many bosses and all kind of unique. Not many enrage timers/ dont stand at this spot- bosses.
I dont know what it is but these days in raid it just feels that you clear some mobs and get to a big room where boss is standing in the middle. In karazhan the bosses fit so perfectly in their places. Liki Moroes in the dinner room etc. Definetly best memories are from there.
Lorewise im not too sure. Never given too much about lore and im not honestly sure what is the story of Karazhan. Gorehowl dropping from prince is kind of messed up too.
Best raid in my opinion. Flame on.