I think when Mike says, "Enough!!" and slams his hand on the desk is a good one. Then when he says "NO", and lies to his wife soooo convincingly. Thats a crucial scene with great "one-worders".

I also believe when Mary says "Dad?" is heart-breaking. It seems to me to be a very real response to what happens to her.

hmmm..what else? The Godfather is full of these things though..like when..oh I can't think of any right now. I swear I must watch this film again, its that time of the year.