Willow. I really had a real problem with Dark Willow, which also actually effected the way I felt towards Willow in Season 7.
And power/battle wise, although Dark Willow would seem to be the obvious choice cos of the 'dark' thing, I'd actually still go for Willow. I think Dark Willow is impulsive and far too controlled by her emotions, but Willow, as her normal self (post badness) is more grounded and logical and I think she could her wicked-self's ass! 😛