Lots of my fave parts have been mentioned but here are a few of mine

-The look of despair and resignation on the face of Tessio when he's been rumbled, in that split second, he knows his life is over - top, top acting. RIP Abe. Plus, the sad expression on Tom's face in the same scene, things have changed and will never be the same again, and when he tells Tessio that Mike understands it was only business, it's genuine.
- The left-handed response of Michael to McCluskey's handshake, and the expression of nonchalance that neither McCluskey or Sollozzo could see.
- The way Bruno's cigarette hangs from his mouth effortlessly while he co-participates in the murder of Luca
- The way Sonny tells Carlo to never touch his sister again through exhaustion.
- The Don's complete indifference to Fredo when he enters his bedroom alone.
- Tom's voice breaking with emotion when he tells the Don that Sonny is dead
- The very blatant smug expressions of Tattaglia and Barzini to each other in the meeting
- Carlo's little grin to himself when he gets in the car, thinking he has got away with it then... 'Hello Carlo'