Tires, hands down. At first they were rolling on wooden (or steel belted wooden) wheels, with no traction at all.
No matter what subsequent safety feature appeared past that, nothing made the automobile safer than having rubber tires.
Just imagine trying to brake or turn on ANY car without tires...

The most important safety feature in cars is a competent, alert driver. Some countries realize this, and take the time to make sure their drivers have experience with many different scenarios and can handle a car in many conditions, so they're not surprised or rattled when something inevitably happens.

3 point seatbelts. Thank you Volvo.

Synthesized engine noise being piped through the speakers. Why bother with this? Why not just make the car sound better? And the worst part is that the BMW M5 is the type of car this is on. That's right, the top of the line twin turbo V8 5-series has fake engine noise as a "feature".....On a tiny hot hatch meant to…

Park Assist. Does anyone actually use this crap?

CVT's with designated ratios. Defeating it's own purpose.