Tough question, 75 years is a long time. Here is a list of candidates, in approximate time sequence:

1) Air Conditioning

2) Automatic transmission

3) Power steering/Power braking

4) Seat belts

5) Disk brake

6) Wax thermostat

7) Fuel injection

8) Emissions controls

9) Intermittent wipers

10) Turbocharging

11) Radial tires

12) Electronics, including EFI, OBD, and computerized everything

But my choice would be Sirius, because as long as you have the right sounds, everything is more tolerable.