Who is your best customer?

My father, who has had an incredible entrepreneurial journey having taken multiple companies from conception to going public, often asks simple questions that have incredibly deep meaning. One innocuous sounding question that he asked me years ago, turned out to be one of the most important questions that has guided my last 20 years as an entrepreneur and consultant. I have also found that when I stop asking this question, things often start to go in the wrong direction.

So here it is:
Who is your best customer?