5*

I would say Anna since she can be really good with the right skills and dedication

Look at mine

Fae would be the most useful in the current meta but she requires some expensive skill inheritance (at least Lightning Breath+). .. For the budget option Anna, Xander, Camus, or Eirika wouldn't be bad. However, at this stage, you might as well wait for the new GHB.

Fir and Sophia usually done at 4* merges... So I wouldn't promote but would instead recommending going for 4*+10.

Xander Camus or Fae are your strongest options here. Mainly because they are all excellent units who can use Distant Counter weapons, Xander and Camus come automatically with them at 5* and so are a good choice, but Fae can inherit Lightning breath+ and is great vs reinhardt and blues.

Arvis when he comes out.

Go with Camus or Xander.