I think Louisa is the smartest of the bunch.

When I first read Persuasion, I was, like the Admiral, unable to tell the Musgrove daughters apart. On this read, I've noticed that Louisa is quite perceptive. She sees and knows to regard highly the warmth and love of the Admiral and his wife as well as the Harvilles affection for FW. She is not merely sucking up to Frederick here (though, I do't supose it hurts that he would surely be pleased by her reflections), as at one point she shares these effusions with Anne while Wentworth is nowhere around. I also think Louisa was behind reuniting Henrietta with Charles Hayter, which not only got her competition out of the way, but won her points with Wentworth.