I don’t know if any particular driving rule or piece of advice could be considered the most important of all. Violating most of them could have dire consequences. That being said, if I had to choose a number one rule, it would be the following:
Rule #1 - Whenever you get into the driver’s seat of a vehicle, before you start it, before you buckle up, this should come immediately to mind. Think “the safe operation of this vehicle is now my number one concern”. All other concerns, and I do mean all other, are secondary. Even if you’re taking a sick child to the ER, your primary focus has to be on driving. If it isn’t, you may both get a trip to the ER or the morgue.
In my opinion, this rule opens the mental door that will enable you to follow all the other rules, and to be observant of those drivers both around and ahead of you, and the road and weather conditions. Really, it’s probably more of a state of mind than a rule, nevertheless it is, I feel, the most important.