Chapter 20 might be the most important in the entire novel. If you haven't read it, you should, if for nothing but the greatest closing argument of all time. This is the scene that contains the famous lines: From, "One way in this country in which all men were created equal...and that is the court of law" to "In the name of God, do your duty."
This closing argument is the pinnacle of everything Atticus Finch stands for, and in a segregated, racist and hateful society, he levels all Americans out on the same playing field - his playing field - the court. It is assumed that when he finishes his speech (even though the jury still convicts Tom Robinson in the end) that everyone knows for a moment how wrong society really is.
I agree that this is THE MOST IMPORTANT CHAPTER. For the reasons above, and because the beginning of the chapter demonstrates how racism affected a child.
Dill grew sick in the end of 19 and had to be escorted out of the courtroom by Scout. Although it was hot, likely stinky, and stuffy, this is not why Dill grew sick. He was physically troubled by watching Mr. Gilmer treat Tom Robinson with such disdain and disrespect. I would imagine most children didn't see many black-white interactions of their parents or caretakers because they were often interacting with their own kind.
This ill-treatment of a human being sickened him. The meeting with Mr. Dolphus Raymond and discussion that ensued proved that perceptions and appearances aren't always what they seem. This is why what Mr. Gilmer was doing was so wrong. He was treating Tom terribly because he was black. Mr. Raymond, who loved the black community, demonstrated to the children that things aren't always what they seem by letting them in on his secret Coke habit. He let the community believe he drank too much (alcohol) just so that they could justify in their minds why a white man would want a black woman... especially a rich white man. This confirmed for Dill that his feelings were well-founded, and encouraged Scout and Dill to return to the courtroom to see the situation conclude. Had he not intervened, we wouldn't have seen the narrator finish her experience in the courtroom and would have received second-hand information.