Well, hm.
I like historical Darla because it's really fascinating to see her as a vampire in the height of her evilness... although that's partially because I'm a costume-buff-in-training (my specialty is Revolutionary War Colonial America, with a minor in Regency Era England). I like seeing what they did with the costuming, and even though some of it sucks, there are some parts that are actually half-way decent considering their budget (Angel's modern tie during the Boxer rebellion is, however, atrocious and unforgiveable). So I guess I like her because of what she wears. 😉
I like pregnant Darla because of the changes that her character takes. And I like the very end, realising that she will no longer have a soul after her son is born, and sacrificing her life for his (vampires love babies, as soon as he was born she'd probably eat him for dinner, I'm betting).
I like white Darla because it shows how much she loves her son and what she was willing to do to protect him from the fate that she received. It was the most moving bit of Darla throughout the entire story, I think. That's why I went with that. Also because I'm a sucker for happy endings and the idea of Darla being loving and caring makes me happy. 😉