I'd say Sabo's father sucked the most at being a father. He's the only one who replaced his son. But Zuko's father sucked the most at being a decent human being, since he tried to take over the world and burn everything. At least Todoroki's father continues to care for him and other people in his own, odd, unsettling way.

Well, Fire Lord Ozai kinda DID kinda throw Zuko out and banished him to search for the Avatar, a quest that he honestly didn't expect to even happen, let alone have his son succeed in doing so. And he DID replace him with Azula, even going so far as to have his daughter capture him and his uncle for possibly fraternizing with the Avatar, and brand him as an enemy of the state… Yeah, Ozai is a pretty sucky dad. I don't know much about the others to make a solid case, though.