Darla and VW are close (and were, generally speaking, more entertaining than my choice), but I'm gonna have to go with Dru as she could be genuinely creepy at times, the way I love vampires to be (as opposed to super villain or evil mutant like characters, or even strangely as figures of empowerment or at least a loophole in which one--male or female--can be a sexualized psychopath and still be swooned over). Like this:
Drusilla: My mummy used to sing me to sleep at night. [sings creepily and off-pitch] Run and catch / The lamb is caught in the blackberry patch... [speaking] She had the sweetest voice. What will your mummy sing when they find your body?
Yeah, that's my favorite kind of vampire, as creatures of horror (not to be confused with simple foils for the heroes) first and foremost, and their own nightmare as well as a nightmare for everyone else.