Edmund Exley's simultaneous interrogation of the three murder suspects: "Sugar, I'm talking about a gas chamber bounce and you haven't even asked me what this is about. You've got a big guilty sign around your neck."
More specifically, Jack Vincennes realizes that he's dying so he uses his final words to trick Dudley into incriminating himself to Exley, which ultimately brings about Dudley's downfall.
Bud White's epic smackdown of D.A. Ellis Lowe, culminating in dangling him by his ankles from his office balcony while Exley looks on nonchalantly.
"Now, I know you think you're the A-number one hotshot. Well, here's the juice: if I take you out, there'll be ten more lawyers to take your place tomorrow. They just won't come on the bus, that's all!"
Capped by Ed's "That the way you ran Good Cop/Bad Cop?"
The film itself. It manages to take a 500 page, unfilmable novel and turn it into a fantastic 2-hour movie and makes it work. There is a lot of Adaptation Distillation, but this film makes it work, so much so that James Ellroy himself loved it.
"Just one hero?"
Inez Soto calling Ed out on his bullshit in the book, flat-out telling him she manipulated him into killing the Nite Owl suspects. The "absolute justice" Ed believes in more than anything isn't his, it's hers.