Calpurnia's best moments do seem to come when she takes Jem and Scout to the First Purchase Church, where the children gain an educational experience about Negro life in Maycomb. Cal makes the decision to take the children herself (we know this because Atticus is surprised to hear about it later), dressing them in their finest clothes to show them off to her friends. Cal stands up to Lula when the woman demands that the white children should not enter. After the service, Scout further bonds with Cal, questioning her about her "double life" between the Finches and her own friends; and asking Cal if she can come visit at her house some time.
Cal warns the neighborhood about the mad dog on the loose and makes sure that everyone stays out of the streets. Her food at the Missionary Circle tea is a success, and she willingly follows Atticus when he asks her to help him break the news about Tom's death to his wife. But her greatest moment does not reflect one specific event: It is the years of service she has given the Finch family, supplying the children with a female presence who is not afraid of doling out daily doses of tough love and inspirational guidance.
Another one would be when she tells Scout how much she missed them after school and when Walter Cunningham came over for dinner and she punished Scout.