Yeah, I'd say Kim...if Venus is off and the surface is slow. Venus owns Kim on hardcourts. I was at this year's BOWC finals match and Venus SHOULD have won it 6-3, 6-1 against Kim. Tour Champs I am not counting because Venus could hardly move there (I was there live as well and witnessed this). I truly think that Kim is the 3rd Williams sister, although she plays more like Serena.
Other than Kim, Mauresmo has a chance at Venus as well. But then after that, no one really has a consistent chance of beating Venus. Sure, there will be the odd match when Venus is off and her opponent is playing the match of her life. But this will not happen on a consistent basis.