Part I: When carlo comes up to hug Michael and Michael says "get out of my sight." Then when he walks away after Clemenza kills carlo.(this is also Al Pacino's favorite scene according to an interview)

Part II: The end sequence- going from Fredo dying to the flash back with Sonny to michael and Vito on the train to finally Michael on the chair. This is the single best and emotional scene ever.

Part III: This is the funniest scene in the trilogy. "Hey joey zasa up ur ass" and he keys the car.