Posts like this never fail to annoy because they always leave someone out, in this case they should have added Callisto from Xena: Warrior Princess.
Callisto is one of the great ones. As a child, her village and family were destroyed by Xena and her army (back in her evil warlord days) and Callisto grew up to vow revenge. Her purpose became twisted with her admiration/obsession with Xena and she proved to be a character of integrity – one Xena couldn’t easily dismiss or dispose of – a living embodiment of her guilt.
As portrayed by Hudson Leick, Callisto was a cunning, single-minded psychopath determined to destroy all Xena held dear. Like The Joker to Batman, Callisto was born to vex Xena at every turn, mocking her turn to good (Callisto felt that, as a villain, Xena was awesome, but as a hero – pathetic), doing all she could to twist her closest relationships (namely with Gabrielle) and destroy the warrior princess from the inside out.
Callisto had a myriad of ups and downs – she was never really able to ‘defeat’ Xena – but she had some fantastic moments (see her introduction and her life as a literal demon in hell) and she was a character who never compromised. A skilled warrior (second only to Xena herself – Callisto is one of the few who could wield the Chakram), unphased by love or a desire for power (she may have led an army for a time, but Callisto’s desires were never for conquest – just destruction), Callisto was Xena’s dark mirror – charismatic, brilliant, witty – and cracked right down the middle.
The Doctor Is Out 