1) The scene where Kay tells Michael that she had an abortion. This is, imo, the single best performance Pacino ever gave.

2) The scene where Michael hears Fredo's slip up in the Cuban sex club and realizes Fredo was the traitor in his family. Another brilliantly acted scene by Pacino. You can actually see the life drain out of him as the realization dawns on him.

3) The scene where Michael is about to kill Sollozo in the restaurant. The way the scene builds is amazing.