I have lots of favorites, and they change each time i watch the movies, but here are a few:

1) see my signature below
2) Vito's speech at the meeting of the heads of the families. "..or if he's struck by a bolt of lightning.." Vito is usually laid back and unassuming, but at that moment you realize, and the heads of the families realize, he is not one to fuck with. Michael never knew what a ballsy thing his old man did for him at that meeting. He threatened the heads of the five families to keep Mike safe.
3) When Vito comes home after losing his job at the grocery store and gives his wife the pear.