Nitti and Ness Scene
(SPOILER, of course)
When Ness pushes Nitti off the roof, that is the most visceral revenge scene I have ever seen.
And, it shows that Costner has that in him as an actor to make that work. You truly feel this good man being pushed too far and cracking.
That is the difference between him and someone like Tom Hanks, who has tried to do dark roles and simply can't do it.