Drusilla is by far my favourite on the list, but based just on looks I'd say Akasha, then Dru, then Darla then Selene.

Add anyone you want because these are the only ones could think of and rememeber the names for (half way through the pick I just developed some stupid mental block with female vamps)

EDIT: I didn't realize I could add. Vampire Willow is just...oh, hell yeah.

vampire!Willow is so hot.

I have to say Darla, i love her unique look =)
Then i would say Akasha, Drusilla, Vamp Willow, Selene :)

I love Selene. She's awesome and very beautiful.

I love all of them so much, it was very hard for me to choose, but in the end, I had to go with Dru. Everything about her is appealing to how she was before she was turned and what happened to her afterwards. She's so complex, if you don't know anything about her, then you just assume she's crazy and needs to die, but for a true fan, like myself who knows about her past, there's a reason she is crazy, she was driven mad. Her life story is very tragic and that's what makes her the most interesting character in my opnion.

I wish the script was better so the movie would be at least good.