I always get a laugh when Connie tells Michael she wants to "book passage on the 'Queen'", and Michael replies: "Why come to me? Go see a travel agent."

Also, Fredo's little "two-step" when he opens the door to Michael's Vegas suite prior to the meeting with Moe Green.

Same scene, Moe's little bit of "Rodney Dangerfield" business with his collar and tie when Michael is talking about buying him out.

The scene on Hyman Roth's hotel room balcony in Havana, when Buschetta removes his hat, looks at the bird droppings on it, and then looks up at the sky. (I never realized exactly what had happened until it was pointed out by Turnbull in another thread).