There isn't a difference here, and you could also use "member of the team". All of these are fine.
However, saying someone is the "best member" is a little strange. It is like saying "she is the best at being a member". Probably you mean that whatever the team does, she is the best at, and while this is implied, it would be better to say what it is that she does well.
"She is the best player on the team." That is, "she" plays (the sport or game) better than the other members of her team.
"She is the best worker on the team." That is, "she" works better than the others.