In one sense you cannot argue that any single event or action had the biggest impact on Scout. Her awareness of the need to stand in other's shoes, that lovely metaphor for empathy, is crystallised by the collective of any number of people and actions, such as what happens to Tom, Boo Radley's coming out, the shooting of the mad dog and subsequent discovery that her father is much more than she and Jem had imagined.... and so it goes.
In another sense it strikes me that choosing, as Harper Lee does, to climax the novel with Boo Radley's saving of the children (with the lovely temporal connective with the beginning of the novel where we learn of Jem's broken arm - a mystery only solves by the novel's ultimate resolution) and to end it with her standing on Boo Radley's front verandah (shorthand for shoes) might prompt one to perhaps plump for Boo Radley's coming out to rescue them as having the most significant impact on Scout's sensibilities. It certainly has at least one obvous impact. If Boo had not rescued the children from Mr Ewell perhaps Scout would have died and thus been unable, metaphorically, to write her story as an adult with a child's sensibilities.