Definitely Kayo's mother because she actually physically abused her almost everyday.. Although Nagisa's mother is bad too, they made up at least..

Nagisa's mother: Emotional abuse, some physical abuse, forces her son to cross-dress, drugged him, possessive, extreme anger issues.

Kayo's mother: Extreme physical abuse, verbal abuse, emotional abuse, drowned her own daughter, doesn't care about her, anger issues.

Honestly, I myself don't even know. >.<

Nagisa's mother @-@

they're both horrible but one thing erased did well was showing how Kayo's mom was stuck in the circle of abuse, and did not condone her actions but took her away from Kayo so that both could heal, assassination classroom not only LET the mom keep nagisa but their rushed conciliated sickened me and acted like nothing happened, assasination classroom needed a few more episodes to fix this

I feel that Nagisa's mother wanted what was best for him, but pursued it in the wrong way. Luckily, they made up and started to understand each other.

Kayo's mom went through a lot and resorted her anger all on her. Though I feel sorry for this mother, I personally find that she's a lot worse >_<

After thinking about it for some time, I think Kayo's mom worse. She didn't seem to care about her child at all while Nagisa's mother kinda did, although in a very wrong way. At least they managed to overcome that.

@Lanalamprouge. It did seem a bit rushed in the anime, but that's because it tried to fit in the rest of the entire manga in it. I feel like the manga did a better job at fixing that and showed the improvement more fleshed out.