From what I've gathered, things didn't really slip until after A Day At The Races (Thalberg's death). And A Night in Casablanca was a rebound from the four before it.
For what it's worth, here's how they are graded on IMDB
Soup - 8.1/10
Opera - 8.0
Feathers - 7.7
Business - 7.6
Crackers - 7.6
Races - 7.6
Cocoanuts - 7.1
Casablanca - 7.0
Circus - 6.8
West - 6.7
Service - 6.6
Store - 6.4
Love Happy - 5.7
On pure plot, Big Store, Room Service, and A Day At The Races seem like the most intriguing but time will tell.
Anyone here dare to rank these films?
I've been a fan of Marx's almost as long as the Stooges. Here's my ranking:
Horse Feathers 9.5/10 (a bunch of classic bits in this one + My favorite Groucho song;
I'm Against It. The Life Savers candy bit may seem worn out, but it was the first time it was ever done)
Duck Soup - 9.4 /10 (many say it's their best, I can't argue, but I've a soft spot for Horse Feathers)
Opera - 9.0 (if it didn't have the musical numbers it would be an even stronger movie IMHO. Groucho's stateroom 'nuff said)
Cocoanuts - 8.0 (Whyaduck?,
I Want My Shirt is a gem. Music by Irving Berlin. Raw and somewhat unpolished, an underrated movie)
Monkey Business - 7.9
Animal Crackers - 7.9 (Hooray For Capt Spaulding)
A Day At The Races - 7.2
A Night in Casablanca - 6.9 (so-so , but easily their best effort of the 1940's)
Big Store - 6.5
Go West - 6.2 overall - 8.0 for the first 10 minutes, 5.5 after that
Room Service - 5.2 (claustrophobic, not really written for them, a few laughs, and a young Lucille Ball)
At The Circus - 3.5 (I loathe this movie, not even
Lydia The Tattooed Lady can help this dog out)
Love Happy - No Rating, because to me it's not a Marx Brothers Movie, worth a watch to see Marilyn Monroe's screen debut in the short Groucho scenes.
DM