I think the funniest moments involve Fredo:

1. At Connie's wedding, a tipsy Fredo sort of introduces Michael to Kay after Michael introduces her to Fredo.

2. His little corny entrance dance into the hotel room in Vegas as the band plays.

3. "I like Havana. Havana's my kind of town. Do we know anyone in Havana?"

4. Congratulating Michael for joining the service.

and, of course,

5. "I'm smart. Not dumb, like everybody says."