I love the Calabrese Landlord in PartII when he goes to see Vito at his office after finding out who he is, and they talk a bit and then the Landlord goes to sit down but quickly stops and then asks for permission to sit. That line is hilarious!

and for some reason in PartIII when Vince shoots Zasa in the back and before he rides off on the horse just says "Zasa" and then takes off. Something about the way he says it makes me laugh.

I know this about funny parts, but the part that makes me cringe is The Phantom Punch Sonny gives Carlo, you all know the one!