Tough one... what are you trying to show them - a realistic depiction of life in that time period or an example of filmmaking and cinema. For the state of cinema I'd have to take a high-concept film in the 1980s so Top Gun would be the likeliest. In the 1990s it would be big-budget so I'd go with Terminator 2, and then 2000s it would be the special-effects-riddled series fanchise so Lord Of The Rings would have to suffice.
For a depiction of life in that time-period it would be a much tougher choice. Probably go for something like Working Girl (1980s), Secrets and Lies (1990s, British life), and...erm...Juno (2000s).