For me personally, it is Ikuko.
I mean, I am sure that Serenity was fine too, but first of all, I only see her as mother of Usagi in her past life, where she seemed like a different person, and second, Usagi, and Ikuko had so many more human moments together, both good, and bad.
They just seemed to have better chemistry as characters, which is funny, considering Queen Serenity was designed to look like older Usagi basically.
I mean, I am sure that Serenity was fine too, but first of all, I only see her as mother of Usagi in her past life, where she seemed like a different person, and second, Usagi, and Ikuko had so many more human moments together, both good, and bad.
They just seemed to have better chemistry as characters, which is funny, considering Queen Serenity was designed to look like older Usagi basically.