I can't believe that any responsible parent has a "favorite" child. Though I am not a parent myself, I still, nevertheless, find the "favorite" child concept difficult to fathom.
Certainly parents might have specific children, in their families, with whom they "identify" with more so than the other children -- but this is NOT the idea of favorites. It is possible, in families, with two or more children, that one or both parents might perceive x child as "just like me" (in terms of interests, basic personality, etc), or "a chip off the old block" even, but again, this is not the idea of favorites.
I would certainly like to think that most parents appreciate each child for her particular characteristics, but you get cases in which a parent perceives x child as "a carbon copy of me," and so forth, but this is not the idea of favorites; it certainly shouldn't be!