It's a difficult question, really.
Nao. She's great, powerful and so on. But after several years she
will retire, I'm sure.
Chie. I like her and I think we could become good friends. So she's a good choice for me.
Natsuki. Shizuru comes in a set, so you can get two otomes instead of one.
Shiho. I believe she would be a loyal and strong otome, you just need to befriend with her.
Can't choose one of them. But I know for sure, that I would never choose Mai as my otome. Who wants to fight with her fan-boys? Not me.