1) When Luca Brasi stumbles through the speech (I'm aware that was a blooper and they kept it in) and the look that Brando gives him is pretty funny.

2) When Don Vito slaps Johnny and Tom is just sitting there in the corner laughing.

3) Signor Roberto apologizing to Don Vito for the apartment problem. Vito and Genco have a good laugh at Roberto's expense.