Though Shawshank Redemption has gotten the critical raves and the box office receipts that could make it the best prison story ever filmed, my personal choice for the best film ever done about the penal system is Brubaker.
Based on the true life experiences of Thomas Murton, the co-author of the book the film is based on, Brubaker captures the realities of prison life, the complexities of trying to "reform" the system without sacrificing one iota of entertainment.
Robert Redford leads a great ensemble cast in this film. The people here are real, dealing with complex issues for which there are no simple answers. The corruption of the penal system runs deep and helped in part by the prisoners themselves who don't want to see too much change at once or have a vested interest in seeing things run just as they are. Yaphetto Kotto and Tim McIntire are two such prison trustees, both of them showing very different reasons why they don't like some or all of what Redford is doing as warden.
Among the supporting cast, I'd like to single out David Keith who was the best as the prisoner whose life Brubaker saves and ultimately becomes his biggest booster in the joint. A really outstanding job by David Keith as this simple country kid who just got caught up in "the system."
Rare you can say a film is informative as well as entertaining, but Brubaker definitely fits in both categories.