Part III: In the meeting between Michael, Joe Zasa and Vincent, I always liked how Vincent angrily demanded the last word in his speech "It's a little foolish of this guy, don't you think? Right? RIGHT?" and Michael is visibly startled, almost jumping in his seat, as if nobody has ever raised their voice in his office before.

Part II: The metamorphosis of Pentangeli's ad-libbed Italian song into "Pop Goes The Weasel".

But I agree, the funniest moment in the whole trilogy is Vito Corleone mocking Johnny Fontane "What am I gonna do?". I think you can actually see Al Martino laughing as well as Robert Duvall (as ScarfaceRH pointed out).

Paul.