12 Angry men jumps immediately to mind. The 1957 Version. Except for the opening scene in the courtroom and the closing scene in front of the court steps the whole movie takes place in a jury deliberation room. Oh yeah, and the bathroom
Besides being a "who's who" of Hollywood at the time, and having an amazing cast (Henry Fonda, Lee J. Cobb, Jack Klugman, E.G. Marshall, and others) it's one of those movies that must have strong performances from the actors, or fail.
In my opinion, it's one of the better non-special effect movies ever made.