Duck Soup
Never saw this one before, but it is a lot of fun. That said, I think I'm not really a fan of Harpo for the most part, at least in solo scenes. He does good physical comedy, but any time I spent with him is usually time I want to see Groucho (but when he and Groucho are together, there's good chemistry). I think what I like about the Marx brothers (save for poor Zeppo) is that they actually have very different styles. OK, Chico and Groucho both use fast wordplay, but even within that they have really different vibes. I mean, I like the 3 Stooges but while there is a bit of difference in their personality, their humour is part of a cohesive whole. On the other hand, the varying styles between the brothers are quite different but act as a great compliment.
I was actually really surprised by the last act of the movie, which is still hilarious but in a completely unexpected way. I was expecting the war that was being built up to, but it was both hilarious and... i really can't call it dark, because it never really looks at the horrors of war save for broken buildings (though the film does imply that people died and while Rufus T. Firefly is a charming, fun-loving leader, Freedonia is a totalitarian state) but they way it's presented is completely chaotic in a way I didn't expect, turning a battleground into a playground.
I think most people feel this is the superior film and I get why, but I might prefer A Night at the Opera. Sure, Duck Soup doesn't waste time with a lame romance plot, but I think I like it more when the Brothers are on the side of angels, whereas in this they are on the side of pure chaos (again, an understandable reason for preferring Duck Soup).