Support your statements with examples or brief quotes from the novel.
Calpurnia would be my first choice. She is such a smart, strong, and influential character in the book. However, an argument can be made for Aunt Alexandria. She is everything Scout learns not to be--haughty, close-minded, and racist. It's hard to believe that that woman is Atticus' sister!
While I agree with the answer regarding Calpurnia, I would have to say that Miss Maudie is a great female figure in Scout's life and is an excellent complement to Atticus.
When Jem and Dill exclude Scout, she begins spending more time with Miss Maudie. It is through Miss Maudie that Scout gets her first factual information about Boo and the Radley family. Miss Maudie clears up the rumor that Boo spies in Miss Stephanie's window (and lets Scout know what Miss Stephanie is truly like), and explains a bit more about the Radley family beliefs--that they are foot-washing Baptists and entitled to their own lives and to conduct business as they see fit. Miss Maudie start the create some sympathy for Boo in Scout's mind.
Miss Maudie also teaches the kids why it is a sin to shoot a mockingbird--the mockingbirds only make beautiful music. They do nothing to deserve being shot. This theme recurs in the novel several times, and it's important to note that Atticus lays down the rule and Miss Maudie explains it.
Miss Maudie can also be, at times, a co-conspirator for the kids. She allows them to play in her yard and gardens, talks to them as though they were adults (like Atticus does) and offers them cake and advice.
I think the greatest female influence in Scout's life is that of Calpurnia, their maid and nanny. She is a steady influence in Scout's life and the closest thing to a mother she has known. Calpurnia (or Cal as the Finch family calls her) is there not only to discipline, feed, and care for Scout, but she is also a role model in that she sets an example for Scout's behavior and freely steps in to correct her behavior when necessary, much as a mother would do. One such case of this is when Jem invites Walter Cunningham over for dinner and Scout exclaims her shock at seeing his horrible table manners. Calpurnia quickly pulls Scout into the kitchen to finish her meal there and explains to her that her behavior to their houseguest was rude and unexcusable. She then goes on to explain to Scout how to properly treat a guest, explaining to her the meaning of her actions:
"there's some folks that don't eat like us... but you ain't called on to contradict 'em at the table when they don't. That boy's yo comp'ny and if he wants to eat up the table cloth you let him, you hear?... Don't matter who they are, anybody sets foot in this house's yo' comp'ny, and don't you let me catch you remarkin' on their ways like you was so high and mighty!"
This shows how Cal is there to teach her right from wrong, especially when it comes to showing and earning respect.