OOPS! Wrong Elizabeth...but we don't mind Mr. Bennet having a favorite in his Lizzie. And he publicly makes comparisons between Lizzie and her sisters that would cause civil war in a family. Even Jane does not compare well with her in his eyes. We accept Mr.B's. sometimes outrageous favoritism. Why are we critical of Sir Walter's ? JA has made us think ill of Sir Walter's judgment (though, apparently, he did all right when choosing a wife). I think it is the degree to which Anne materially suffers, her helpless dependence on good will which she deserves but barely receives, that make Sir Walter's favoritism odious. Unlike Elizabeth Bennet, Elizabeth Eliot takes full advantage of her position with her father to shut her sister out.

BTW, I don't think we have any indication from the text that Lady Eliot played favorites with her daughters. We may (perhaps incorrectly) infer that from Lady Russell's love for her deceased friend, and her attachment to Anne. We may also be imagining a parallel relationship as we see Sir Walter and Elizabeth in action. But mostly, WE have such a preference for Anne that we cannot imagine BOTH her parents could have undervalued her.(And I also think we have a sympathy, if not a liking, for Lady Eliot that makes us think she MUST have loved Anne best!)