I'd go for Ma Corleone's wake.
When Michael walks through the room, up to Fredo and hugs him, the camera showing Fredo desperatley clutching Michaels back, and then Michael gives Al Neri that cold classic Pacino stare, signaling Fredo's fate in a glance. Also the musical score is a perfect accompanyment to the scene. This for me, is a classic Godfather moment.