Hi Grasscarp, I didn't answer directly because I thought it would have been covered in the previous response. However:
A original questioner can choose a different best answer if they wish. The Best Answer pod swaps to the latest chosen in that question.
"It gives one person the best and everyone else nothing at all."
This is sort of the definition of "best"?
As I said previously, we have to accept that like most things in life, this is going to be a fairly subjective choice.
"Sometimes one thread has hundreds of responses - is only one "best answer" really appropriate?"
Yes, it is on these threads which the best answer is most appropriate, a new poster visiting will be able to jump to the piece of information deemed most useful by the questioner.