As said before, it depends on the game. I have always felt that in most games it was Zelda (or Tetra, in the case of games where Zelda is Tetra). However, in
TP I think Illia is the best choice, even though she makes me mad because she jumps to conclusions, seems to have a hard time taking care of herself, etc. In MM, as there was no Zelda, I think it would be one of the sisters, either Romani or Cremia. I forget which is older, but the younger would be good because she is around the same age as Link, the other just because of the hug she gives Link after your second time doing the milk run mini game.
So I guess over all, Zelda would be the best choice, as she takes the cake in most of the games.