Baraggan
Louisenbairn

Baraggan Louisenbairn ruled Hueco Mundo before Aizen arrived. He held Las Noches as its God-King, kept an open court and an army of Hollows, and was so completely without a rival that he had grown bored of his own throne. Aizen walked in with Gin Ichimaru and Kaname Tōsen, demonstrated the gap between them, and offered him a place as the Segunda Espada. Baraggan took it and never forgave it.

His aspect of death is old age, and his ability is the purest expression of that idea in the series. Respira is a black miasma that does not burn or cut but ages whatever it touches until it crumbles, and it spreads through anything it makes contact with, including an attack aimed at him and the arm that threw it. There is no defence that works by being harder or faster.

Hachigen Ushōda beats him by not trying to. He seals the ability inside a Kidō barrier, then transplants Baraggan's own decay into it, and the King of Hueco Mundo dissolves screaming that he is eternal. His last act is to throw his axe at Aizen, who does not bother to look up. That is the whole relationship in one gesture.