Meritocracy is almost everything in Fortune 500 C-suites. Top college, MBA or JD program plus experience in top publicly traded companies or big finance/banking. The exceptions are for companies that need technical expertise, like Oil and Gas or Pharma. This is because search committees are all about CYA. If someone does a lousy job, the search committee wants to be able to say "but this guy/girl had top education and work experience. We hired the top candidate" instead of actually taking the time to get to know the people they hire and give less credentialed people a chance.
There is plenty of nepotism, but it is usually indirect. People get a foot in the door because someone on the search committee worked with their father and or went to school with a family member. There is a lot of "if you scratch my back . . ."
In the end, a lot of it is just luck. People who are hot commodities in the C-suite world are people who can say they produced big results when at Globex. But in reality, the rise and fall of most big companies is purely dependent on macroeconomic conditions. The C-suite people generally can only screw it up and rarely make that much of a difference.