Who has the most positive and most negative influence on Scout?
You seem to be asking for a positive influence and a negative influence on Scout.
AUNT ALEXANDRA seems to be her most negative influence. Scout can pick up on the fact that she doesn't necessarily support Atticus' position through the trial. The missionary tea circle that Scout has to go to (I am sure under the command of Aunt Alexandra) further rubs Scout the wrong way as she sees racism, hypocrisy, and sheer rude behavior dressed up in Sunday's best. Although I believe Aunt Alexandra loves Scout and Jem, and she comes to help, Scout the narrator paints Aunt Alexandra as a person not to copy.
MISS MAUDIE teaches Scout values through example and respect. When the boys are rude to Scout in chapter 5, Miss Maudie takes Scout under her wing and determines to enlighten her about neighborhood history, shadings, and the value of her father. Scout learns to respect and admire her dad for doing right even when no one else wants him to because Maudie points out what a good man he is. By the time the missionary tea rolls around, Maudie is there to keep Scout from acting on her emotions, something that has always been a burden to Scout. Maudie was Scout's most positive influence.
Atticus has the most positive influence on Scout throughout the story. Her father teaches her valuable life lessons and encourages her to become a morally upright individual. Atticus teaches Scout the importance of perspective, tolerance, courage, and justice. Atticus is always honest with his daughter and raises her in a loving home. Scout trusts Atticus and follows in his footsteps as she matures into a morally upright individual.
Aunt Alexandra has the most negative influence on Scout throughout the novel. Alexandra encourages Scout to not interact with poor white children and African Americans. She continually ridicules and criticizes Scout for her lifestyle, which greatly upsets Scout. Scout is forced to suppress her anger around her aunt but cannot always control her negative emotions. Aunt Alexandra forces Scout to lash out at her on several occasions, and Alexandra attempts to instill in Scout prejudiced beliefs.