Sarah Michelle Gellar except when she had that stupid hairdo in season six. Totally unappealing.
Also think Amy Acker is second-best, followed by Charisma Carpenter during season two of Buffy and season one of Angel. For some reason, I don't think she looks as good in the other seasons, especially the second half of season three of Angel onwards. Urggh. What happned to her?
Honorable mentions go out to Sarah Thompson, Julie Benz, Emma Caufield during seasons three and four, and Eliza Dushku.