^LOL Buffy could barely save America.
While Sailor Moon can save the entire solar system or the entire galaxy if she wanted to.
I think it really depends, in terms of the physical fighting skills, then Xena would be the most skilled fighter out of them all I think. But in terms of power, then there's no doubt Sailor Moon is the most powerful fighter out of them all BY FAR, she has the power to save the entire galaxy or even the universe in the form of Sailor Cosmos (at least according to the manga).
Wonder Woman is a way better fighter then Xena she was raised as a warrior from birth and has immense powers as she is blessed by the Gods. I think that Wonder Woman of the 3 would be the best fighter but I would say that Sailor Moon would probably be the most powerful.