There is the beginning, Alex Kitner's death, the two guys on the dock. These are effective scenes. No denying their power. For my money though, one scene outshines them all: Quint's Indianapolis story.
The set up for it is classic: trading stories about battle scars and singing while on a boat in the middle of the ocean. And then, the old man speaks. Quint delivers the story in the kind of way that an old sage would to two students. Their hanging onto every word and Hooper's reaction tells us everything. "Didn't see the first shark for about an half an hour." All the while Quint tells us what happened to the USS Indianapolis, we hear the waves lapping up against the boat. It amounts to a fear and an experience that none of us would want to have. To be stranded in the middle of an ocean and see a shark fin coming toward us. It's as poignant and vicious a segment in Spielberg's canon or just about any other director's canon.