Madara.
Obito was never quite as insane as Madara was. They're both ruthless killers but I get the feeling that Obito might show Madara mercy if he ever bested him. Madara on the otherhand, might feign giving up before he'd stab Obito in the chest/heart.
...The stab in the chest is because of symbolism XD the symbolism that an uchiha's DARKNESS is too strong and so is their heart!!11