It really is a hard choice.
I mean, Nightmare Star has elegance and that beautiful horn crown. But Daybreaker has an awesomely vicious attitude and that sweet armor.
I think Daybreaker wins in the end, though, because of her name. Nightmare Star is a good name, and goes along with the Nightmare naming convention for evil alicorns. But Daybreaker is possibly the most metal name you could come up with for a character like this.
That's the kind of name where you know the writers were all sitting around, workshopping names for the evil Celestia. Then someone said "How about Daybreaker?" and everyone in the room went "Oooooh, shit, that's it", and they all threw their laptops on the ground and went home.