I've mentioned before, but I like the following:

1. Sonny's kid punching Clemenza in the gut during the scene in which Vito arrives home from the hospital and is being taken upstairs.

2. Tom's laugh when Vito mocks Johnny in a girlish voice saying "what can I do?"

3. The icy look Michael flashes at Geary during their meeting in Michael's office. It is priceless when Geary points that decorative canon sitting on the desk at Michael. During that exchange, I always imagine Michael thinking "do you have any idea what I am going to do to you?"

4. The exaggerated Italian accent Clemenza uses toward Michael after Kay calls. "I love-a-you will alla my heart. If I don't-a-see you-a soon, I'm-a-gonna die."