Margin Call is a pretty good candidate, considering how tight its cast is.

Edit: come to think of it, if you want a big cast of the most highly-connected actors, you'd probably pick Cradle Will Rock from back in 1999.

https://en.wikipedia.org/wiki/Cradle_Will_Rock

A ridiculously great film you should see, about everything that matters, all in one modestly history-distorting bonkers movie, with some songs and some cracking dialogue:

  Marc: I am faithful to the ideals of the party.
  Orson Welles: I am faithful to the party of ideas.
  John Houseman: You are faithful to the idea of a party.
  
  John Houseman: As the producer, I can fire anybody I want, and I am fucking fired!