I'm adding my top ten choices for Part II. Many of them have already been mentioned by others.

10. Al Neri lowering his eyes (he's ALMOST showing some humanity) when he's watching Mike embrace Fredo at Mama Corleone's wake. Neri knows he's still going to kill Fredo:



9. A young Vito realizing that Fanucci is not as powerful as the neighborhood thinks:



8. Kay's look of disgust and anger (as she's looking at Michael) after the Tahoe assassination attempt. You can almost feel the contempt:



7. Johnny Ola's smug look after Mike got "dressed down" by Hyman Roth (about who killed Moe Green). You almost want to punch out Ola:



6. Fredo proclaiming that he's smart. One of the saddest (but best acted) scenes in the movie. Fredo's look of weakness was excellently done by Cazale: