I'd pick
Christiana without hesitation, but I'd never use it, not being
Christian myself and not expecting my children to follow that or indeed any religion.
I've never seen
Christa and I'm hoping I never will. It looks like an amputation.
Christina is blandly pretty. I prefer the sound of
Christabel, but I wouldn't use it either. I've got two great-grandmothers named
Christiana, one with no mn and the other was
Christiana Margaret, which is why I would pick it, but the appearance and the meaning are why I'd never use it.