It would be very difficult to say about Kendra, since we actually saw so little of her fighting technique. Buffy and Faith, during their time together in Season 3, were pretty closely matched, as was illustrated by their big fight after Angel was poisoned. Buffy had to use a knife to actually beat Faith. Maybe she could have beaten her eventually, but we'll never really know for sure. The next three years gave Buffy time to build her experience and perfect her technique, something that Faith, obviously, couldn't do in prison, so from that point through Season 7, Buffy was the superior fighter. Beyond that point, given the time to get back in practice, Faith may close the gap once more, provided that she ever sees any real action while babysitting Angel.
(What? Somebody was expecting a short answer from ME on a subject like this?)
Okay, short answer: Buffy's the best, simply because she is the central character in the series. Joss dictates it that way, and the writers write it that way. Sarah's stunt double can beat up Eliza's stunt double. Just as long as the script says she can.