From a gameplay perspective I just loathe Hakkar. With the right setup it's a breeze but it often becomes a straight up mess...
From balancing perspective It would be most of the ones that have heavy passive ticking damage. Tarvold and Beryllia are the biggest culprit. Going up in keys on these ones are just stressful since there's little you can do, you just hope you have the numbers. Mistcaller have the same issue, but I like that fight because it's not the same fight every time. You actually have to pay attention to the mini-game.
But my vote goes for who I dislike from a gameplay pov. If I like a fight and it's unbalanced it's not as bad as if I dislike a fight but it's balanced. Enjoyment is most important to me.
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I really wish Blizzard someday updates the camera to become better. That fight illustrate a problem quite clearly.
Though as a mage, that fight is a boss without mechanics