Out of these, honestly, Kazumi.

I don't get why, she was going to kill Kazuya

Yeah, but for good reasons. Kazuya had the Devil Gene which corrupted him and sometimes, you have to kill those that you love to prevent them from progressing towards more evil.

Sorry, but I don't agree or see the logic.

About the (maybe) necessary sacrifice, I mainly remember that she married Heihachi as a way to kill him, and worse, she didn't do it soon, I don't know if I can blame her for the lack of contraceptives because back then, they must have been limited, especially for women. Her love for Heihachi is implied not to be real, and she is implied to be a fake person in general. She probably had feelings for Kazuya but it makes me wonder if you can love someone and aim to kill them at the same time (I think that love implies you want to protect the person) and also because death is not the only restraint, especially not for a child.

Even Nina seems to avoid killing Steve or having him killed. Despite she in general isn't feeling or kind, and her prologues say that she felt nothing for Steve, and she was moving forward with her job of killing him, the cutscenes betray the descriptions. I saw a YouTube user commenting that she probably didn't mean it when she said he was a thorn in her side, and that she only pushed him away as a means to protect him, because having him around her would only put him in danger. Even though I usually don't believe in Nina's goodness, this comment sounds very believable.

We know little about Michelle, but she is said to have been a good mother, and nothing negative is revealed.

One of the reasons why I love Jun is because she is the best mother someone could have, kind, forgiving, resourceful, protective, and reassured Jin when he did no longer believe in himself while providing him the cure. I never saw anything negative about her (well, at most her sending Jin to Heihachi, which was probably done out of despair and perhaps ignorance about Heihachi).

Thinking about deeds and especially comparisons, I don't see why Kazumi would be considered a good mother, let alone the best.

Fair enough. Ok, Jun definitely is the best Tekken mother.

Jun Kazama