Not the first, but most certainly the one looked back upon by rose colored glasses the most.

Onyxia's lair is in a volcanic cave in Dustwallow Marsh. The floor of the main cavern is just solid enough that Onyxia can sit on it normally without cracking the floor and revealing the lava below. This is intentional; if seriously threatened, she shatters the floor, exposing the lava to further damage her enemies. She has over one hundred eggs in her clutch, but her mate is unknown.