I love De Niro too much and these choices are too strange for me, I can't do it. Not the brother/wife breakdown in Raging Bull? Not the talk with Peter Boyle in Taxi Driver? Not the final reveal scene in Angel Heart? Not the trial-by-God on the boat in Cape Fear? Not the improvised scene in the back room with Harvey Keitel in Mean Streets (and/or the pool hall fight scene)? Not the rescue scene at the beginning or the 12 steps scene at the end of Men of Honor? And WTF are The Mission and The Score even doing on the poll? I'd rather see the polygraph scene from Meet the Parents or a therapy scene from Analyze This, or even the baseball finale of The Fan.
In any event, for me it's when he loses his shit on his wife and brother in Raging Bull. That performance is the GOAT acting performance from the GOAT actor, but that whole sequence, from the "What's with this kissing on the mouth?" to him staring at the scrambled TV after assaulting the only people who inexplicably still care about him...that's not just what greatness looks like, that's what the greatest looks like.