Two of my favorites:

Outside the hospital with Michael and Enzo. Michael poses Enzo up to look like a gangster and together they wait. A far away shot near the street shows Michael in the light and Enzo in the shadows, looking like a silhouetted gangster. He could've been the reincarnation of Luca Brasi for all those guys in the car knew. For a few moments, he wasn't a guy who made pastries and cakes for a living. Brilliant stroke of fortune for Michael that Enzo turned up when he did; Enzo's bravery to stand with Michael was hardly matched anywhere else.

In Cuba where Michael and Fredo are watching the Superman show and Fredo effectively rats himself out by talking about Johnny Ola. The look on Michael's face, as though someone just pulled the world out from underneath him was powerful. The very moment he identified the traitor...and it broke his heart.