Diana, all the way.
Dalia will spend her entire life removing the h from
Dahlia, and
Davina looks closer to Divine than to
David. Which is (a) wrong and (b) over-ambitious. Anyway, I like
David far too much for a boy to ever use a form of it for a girl.
Diana has elegance and history. It also has the late princess, which I find unfortunate, and the nn
Di, which I find ugly. So, mn only.