Drusilla - creepy and hilarious, fully gothic and tragic as well. She is everything a vampire should be.
I like Darla well enough but it frustrates me that she becomes yet another mystical pregnancy on Angel (Angel has way too many mystical pregnancies and they need to stop going to that well). It's treated as part of her character arc and her redemption ... but it isn't at all. It's not anything to do with her. She is possessed by the soul of the baby inside of her, just like Cordelia in season 4. She is no more herself when she stakes herself than Cordelia is throughout season 4. If she had survived the birth she would have reverted back to evil Darla, just as Cordy reverts back to herself in 'You're Welcome'. It's the exact same story line - but moving the characters in opposite directions on the good-evil spectrum. And if Cordelia's mystical pregnancy doesn't represent a fall from grace and and into evil for which she must atone, because she was possessed by the baby so it wasn't her fault, then Darla's possession by Connor's soul does not count as redemption. It's just another (another! they do this so often on Angel) violation of a female body by an unwanted parasite that forces her to act in a way contrary to her own character, her own wants and her own needs. But then it's wrapped up with overtones of the redemptive quality of motherly love ... it's actually very troubling.
And once you remove her 'redemption' arc ... she's OK, but she doesn't really stand out to me. Her main motivation for doing anything throughout season 1 of Buffy and season 2 of Angel does seem to be ... well, Angel.
Harmony - on the other hand - is hilarious and I love her. But she is a terrible vampire. Drusilla FTW.