The Superman scene is one of the most emotional scenes I've ever had the pleasure of watching. When Fredo stupidly remarks "Johnny Ola knows these places like the back of his hand...old nam Roth would never come here", or something to that effect, the look of anguish on Pacino's face is heartbreaking. It's at that moment that Mike knows it was Fredo, and it completely tears him apart. I also thought the ending of Part II ranks up there. Now the most heartbreaking scene for me would be the "Silent Scream" at the end of Part III. It's the only scene that gets the eyes welling up for me. It's especially difficult, because Michael has always been a man who is in control, and now a tragedy has befallen him, and he has absolutely no control over it, it's completely out of his hands, and there's absolutely nothing he can do about it, so he screams. Absolutely heartbreaking.