I don't necessarily think that that pure skill has anything to do with who is the best fighter. Generally, a trained fighter would win in a competition with a untrained fighter, but with Buffy and the Slayer its different.
Faith gave Buffy a slight run for her money, and she had no visable training compared to Buffy.
I think what makes her so special, besides the support group, is to see things differently. In battle she uses her environment along with her skill and power. Shes brilliant and well versed, this is what makes her the best.
I do think in a battle between Xin Rong and Buffy, that Buffy would come out winning. Buffy has proved that she can compete on martial arts levels (look at certain eps) and she is probably the most agile of all the slayers (Even though Kendra and Xin Rong were gymnastic-like, but Buffy in Touched blows them away)...
It is odd how everybody always loves Buffy and favors her in battle, but thats because shes proven herself against so many challenges and shown to be so incredibly strong, physically and emotionally and in the end, she'd do anything to save the world.