I think Lizzy is Mrs. Bennet's least favorite child for the following reasons:

a. She is the child least like her mother. She reads extensively and does not gossip. She is so different from her mother that Mrs. Bennet cannot understand her (Lizzy's) character. Contrast that to Lydia who is Mrs. Bennet's favorite child. Lydia is loud, brash, man crazy and pleasure loving. Mrs. Bennet surely sees Lydia as a kindred spirit.