Obviously the answer to this question will be different for each reader, and I am certain your teacher wants to hear what you think was the most effective event in the novel. For me, the most effective moment (event) in Erik Larson's The Devil and the White City is when the White City is first revealed to the public.
"At precisely 12:08 he touched the gold key." First one engine starts, then "thirty other engines in the building began to thrum." Soon the pumps feed the water through the pipes and
millions of gallons of water began surging through the fair’s mains.... An American flag the size of a mainsail unfurled from the tallest flagpole in the Court of Honor, and immediately two more like-sized flags tumbled from flanking poles, one representing Spain, the other Columbus. Water pressurized by the Worthington pumps exploded from the MacMonnies Fountain and soared a hundred feet into the sky, casting a sheet rainbow across the sun and driving visitors to raise their umbrellas against the spray. Banners and flags and gonfalons suddenly bellied from every cornice, a huge red banner unscrolled along the full length of the Machinery Building,... [and] men and women [had] to shield their eyes. Two hundred white doves leaped for the sky. The guns of the Michigan fired. Steam whistles shrieked. Spontaneously the throng began to sing “My Country ’Tis of Thee,” which many thought of as the national anthem although no song had yet received that designation.
What a glorious moment that had to have been for those who envisioned and created the White City. The dazzling white buildings set against the shoreline of the dark lake, the dazzling electric lights which so many of the crowd had never before seen. The wonders of the White City, both on the inside and the outside, must have been even more amazing to the visitors who had no idea what to expect, as Larson describes them weeping at the beauty they see and experience.
This grand moment must, at times, have seemed unachievable to Daniel Burnham, but the visitors only saw the results, a triumph of human invention and innovation.
“Beneath the stars the lake lay dark and sombre," Stead wrote, "but on its shores gleamed and glowed in golden radiance the ivory city, beautiful as a poet's dream, silent as a city of the dead.”
Of course this is only half the story of the Chicago World's Fair. A man calling himself Doctor Henry Holmes is using this city and this spectacular event to pursue his heinous, deviant behaviors on unsuspecting women. In fact, as the fair opens and the crowd is spontaneously singing, Holmes is already at work.
It is this moment, when the White City is revealed, which most demonstrates the stark contrast between the glistening achievements as well as the dark depravity of the human mind and body in this one place. It is the contrast in this moment which makes this the most effective event in the book for me.
In the end it is a story of the ineluctable conflict between good and evil, daylight and darkness, the White City and the Black.