I always liked the onyxia encounter. It really gave you kind of a sense of awe first going in there and seeing her rise up from her slumber and come to life attacking you and your raid. And everything in the encounter just made sense you know, it felt very organic. She had whelp babies that attacked you, she would fear you with her frightening roar, she'd breath fire all over you, etc.
Most raid bosses are self-defeating. They'll have some ridiculously overpowered move that will 1-shot you or close to 1-shot you, but then ALWAYS provide a means to avoid it themselves like the ice blocks + AOE you can LOS behind the ice blocks on sindragosa and saphiron, etc. I always thought that was so stupid and just bad design to have a boss so idiotic it self defeats like that, and so many of them do it... Killed by their own mechanics. Just feels so unimmersive and ridiculous I can't enjoy it.
Onyxia however was a good fight.