I think all the ones that have been mentioned are great. But I also love the scene with Signor Roberto, where DeNiro just waits silently for the landlord to change the monthly rent, THEN offers his hand. Or when he plays dumb with Clemenza, first when he swears that he didn't look inside the "package" that Clemenza threw him, and again when he pretends not to know that they're stealing the rug.