The funniest Robin Williams movie is Mrs. Doubtfire. In this movie, he takes the role of an old nanny in order to take care of his children who are living with his wife with whom he has had a divorce. Perhaps this character that he plays in this film is what makes it one of his funniest movies.
Moreover, the way he has immersed himself in the character of an old nanny is yet again impressive, and extremely funny. Apart from that, the setting of the events and the way they turn up in the movie add in to the overall experience.