A Night at the Opera, A Day at the Races, The Big Store, and A Night in Casablanca - all of which were made after Zeppo's departure. The youngest Marx Brother was essentially a straight man to his three much more talented siblings, and he just didn't have the personality to stand up to their madcap antics. Thus his leaving not only didn't harm the brothers as an act, but may also have succeeded in sharpening their comedy.
In order to appreciate Duck Soup to the fullest degree, you have to be careful to avoid the preconceived notions that many film critics attach to it. Despite what they may say, Duck Soup is NOT a brilliant satire on Nazism - there are many other movies that fill this role such as Chaplain's The Great Dictator and Ernst Lubisch's To Be or Not to Be, but the Marx Brothers neither try nor inadvertently succeed in making a film that's goal is to lampoon Hitler. It's easy to want to see their work in that light because of their Jewish heritage and the fact that they are indeed satirizing politics and occasionally taking shots at fascism, but anyone who comes to this film expecting a strong, humorous offensive against the Third Reich is sure to be disappointed.
That being said, this is still a very good performance by the Marx Brothers, with Groucho stealing the show as usual. In this one he plays a fast-talker who is given control of the country of Freedonia with predictably disastrous results. Everyone always seems to remember the iconic scene where Freedonia declares war, and then the brothers sing about it. It really is a clever sequence, completely over-the-top and all the better for it. Other than that, it's pretty typical fare with Groucho wisecracking, Harpo doing slapstick, and Chico making immigrant slurs in a bad Italian accent. Still, the comedy doesn't feel stale, so it ends up working out rather well.
I give this film 7.4 out of ten, a fun a silly comic romp.