Of the characters listed, I would have to say that, by the end of the "Angel" series, Spike would have to be the best fighter in the Buffyverse. My reasons for saying it are simple: by that time, he had either beaten or fought to a standstill every other fighter mentioned, with the exception of River.
He almost beat Buffy the first time they fought, and it was only through the fortuitous intervention of a mom and a fire axe that our favorite Slayer wasn't either killed or turned right then.
When there was something that really mattered to Spike (the Cup of Eternal Torment, combined with its prophecy), Angel, by his own admission, got his ass kicked.
No, he never fought River, for obvious reasons, (though, immortal vampire and all, if we accept that the Serenityverse is part of the Buffyverse without the demons, they could square off someday, especially if you accept the theory that River is in some way connected to the Slayer line).
Now, if we were to include Illyria in the list, especially at here peak of power, well...