Who are Hades's enemies?

Answer 1

Basically everybody on Mount Olympus doesn't care for him. Plus he fought his father in the War of the Titans so he's not much of a family man.
Hades did not have any enemies as he rarely ventured from the Underworld or took a interest in matters of the realm of mortals.

Answer 2

He did not have a rival unless you consider that he seemed not to care that Persephone loved Adonis; his enemy and the enemy of his brothers were the Titans.