Tenka is always best girl, and I'm being serious here. She's the one that cares the most about Yuuki, even as everyone else wants to be the commander.
But let us be honest. All of us who read this manga at this point are only wondering about one thing:
Which ones of these girls will start to thirst after MC and join the would-be harem?
Because you DO KNOW, with this amount of girls and in a manga like this, some of them will go after Yuuki.
Ren, the commander in chief, for sure. She's being so much of an ass, and she's obviosuly such a nazi bastard (look at how the third chief fears her) that she will need saving, and then go full yandere on yuuki. Or maybe not, but she gives me that vibe. Yeah, that Esdeath vibe.
The chief of the second, Mira, is also a possibility. She seemed pretty caring, if a complete biker gang boss themed girl.
... of course Fubuki going after Yuuki would be the incredibly funny. Aside for being the super hot milf, just IMAGINE how Himari and Yachiho would react to their mother going after Yuuki. It would be enough comedy for months. Yes I'll admit I pretty much want to see this.