Idk how you are defining “classic” or “older”; Jurassic Park is now 31, for instance.
If it’s B&W, The Killing. It’s a tight 90, engaging heist, Kubrick never misses.
If it’s just 50 years old, The Taking of Pelham 123. Also a heist, also engaging.
What single classic film would you recommend to someone who never watched older movies?