12 Angry Men (1957)
For an iconic film such as this one, it's pretty impressive that this was director Sidney Lumet's directional debut. This was filmed nearly entirely in one jury room and was able to keep the attention of the audience with incredible storytelling and scriptwriting. It's a slow-going story, but the end makes it all worth it.
The film is about a group of 12 jurors who need to make a decision on a death sentence. They must come to an agreement that the defendant is guilty beyond a reasonable doubt, but only one juror casts doubt. The other 11 must convince the 12th juror to agree with the guilty verdict.