For me, the most riveting scene is found in Chapter 18. Scout, Jem, Dill, and arguably even Atticus, and especailly Tom, learn the hard lesson of life, that innocence and justice may not always prevail. This Chapter exposes the lies of the accusers, allows Scout, Jem and Dill to see Atticus both as a professional and as a human being. Further, we begin to feel the exhaustion Atticus feels in battling a corrupt culture.
As for Atticus, he's been in the system long enough to know what an uphill battle is to fight against entrenched prejudice. While he has to take the lying Mayella down, he knows her ignorance and compicity in the rime of false accusation is just a cog in the wheel of the corruption of the culture of hate. He does his job, but after he shreds Mayellea's claims, "(h)e sat with his head down."
The children are also changed in this chapter. They see their father at his most professional, exalt in his exposing of Mayella's lies, and witness his pain as they too begin to understand the insidiousness of prejudice.