Gold Diggers of 1933 (1933)
The plot is simple. Brazen ladies don't let dire economic circumstances get them down; instead go around being fabulous and witty. That's all the plot you need, as it turns out, because everything about this film works. The songs are great, the choreography is wonderful, the dialogue is snappy, and the performers are charismatic. Ruby Keeler is still a terrible singer, though (clip not the same film, but you get the idea). I don't understand how someone can be so terrible and charming at the same time.
The film also offers an interesting alternative glimpse into depression-era attitudes. Modern day depictions of the Depression tend to romanticize certain aspects while overlooking important others. Gold Diggers of 1933 addresses these issues frankly and presents an outlook that is both realistic and positive. Artifacts from the time period itself are more valuable than any re-creation.
In sum, fun, funny, smart, so on. Go watch it right now. Start with this.