[x] ...Neianne.
I know, I know how it looks. But this is character based, honest!
The logical choice obviously comes down to either Neianne, or Sieglinde.
On one hand, Sieglinde would be a good choice because she doesn't want leadership, and she needs to start taking responsibility for her actions, or lack of actions. She may not want it, but the others won't let her slack off and can help her grow into the leader she really needs to be to start implementing all these high ideals she keeps talking about but not doing.
On the other hand, Sieglinde has already had the chance to learn about leadership and being in charge and all that, so it's not exactly something new to her. She's had a chance to learn about it and should be prepared for leadership in theory if she ever gets put into it.
Neianne hasn't however. And she's been showing alot of potential, but because of her 'shy uneducated dryad' self image, she hasn't realized all the pros she actually has. Putting her into a situation where she has to be leadery will expose her to ways of thought and practice she may not have realized, and find out her skills actually kinda fit here.
She's friendly with a lot of noble squad leaders, and gets along with pretty much all of them. I assume squad leaders are going to be dealing with each other in some fashion, and the other options will rub people the wrong way all the time. Neianne may not get the implicit respect Sieglinde would, but she has gained the acknowledgement of the academy at large, and is capable of having a conversation about a contentious topic without pissing the other party off. Unlike Sieglinde by accident, or Elizabeth on purpose.
Second, her unconventional thinking. I'm sure she's not the only one to think outside the box, but having to deal with the leaders and planners will help her refine that sort of plotting and planning.
Thirdly, it would be entirely hilarious to have Neianne walking around being nice while Sieglinde and Elizabeth flank her like terrifying eleven noble bodyguards. I know they aren't actually going to follow us around like a highschool clique.... most of the time. But it'll happen eventually where Neianne will be asking someone for something without a hint of malice, while Elizabeth and Sieglinde loom over her shoulders. And how can we not have that?