Job from the Old Testament
Actually pretty much any character in the old testament is pretty bad.
Dio Brando is a pretty awful dad, although his own father might be worse.
Kanae Ohtori's parents are pretty bad in Utena. Letting a sexual predator marry your daughter and gain control of your school is pretty bad, and it's heavily implied that Kanae's mom is sleeping with the predator as well.
Son Goku, as previously mentioned, is awful at being a parent, but honestly that's just because Goku is awful at everything except fighting so I don't think that counts.
Gendo is a terrible person in a terrible world being the best parent he can be considering those two truths. Get in the f***ing robot, Shinji.
Shou Tucker is an obvious choice. The worst thing about him isn't that he made a monster out of his own daughter, that's just the climax. The real tragedy of the Tuckers is that Shou's whole life was a self-defeating farce. He was out of his depth and faked it to get by, and was praised for it even as he destroyed everything around him. Shou is a rotten parent because the State is a rotten parent; because Bradley is a rotten parent (figure). He makes us wonder if Hohenheim is rotten too.
Nobody talks about Attack on Titan these days and I'm not caught up completely on the manga but I get the feeling that Eren Jaeger's dad is pretty bad, but honestly he's kind of a Hohenheim knockoff.