Three more:

1. The look on Tessio's face when he realizes that Hagen is on to him, as they are standing around the cars. Tessio has this frightened and resigned grin as it sinks in that he has been discovered and will surely be killed.

2. Michael when he looks with subdued fury at Moe Green right after Moe says he had to "straighten out" Fredo. This is right before Mike says back, calmly but with anger, "You straightened out my brother?"

3. Vito's look, and his tear of joy/relief, when Michael tells him solemnly at the hospital, "I'm with you Pop."