In Season 2, when Buffy and Kendra first meet, I'm going to go way against the grain here and say that I think Kendra would beat her. Before you all start lynching me here are my reasons.
In season 2, Buffy was not yet the slayer she would become she was resisting Giles' teaching in a lot of ways, Kendra on the other hand had been being trained since she was a young and was rather accomplished. Kendra pretty well took care of Angel for instance, locking him in that cell and all.
In the first couple season it is important to remember that the only reason Buffy survived on most occasions was the Willow, Xander, Giles etc. had her back. The whole cool little concept being that Buffy was different then most slayers because instead of not having friends her support network was the very thing that kept her alive. It wasn't until later seasons that she truly came into her own as a solo fighter in this mans opinion.
So put me down 1 vote for Kendra the Vampire Slayer.