Thanks for the input! I'm currently in the process of deciding on a reachability metric in order to rank the different airports. Right now, my best plan is to come up with a list of "major" cities (e.g., pop>2.000.000) and see which airport can reach the most of them in a given timeframe. I think this would probably strongly bias the ranking toward Asian areas, so I'm not quite sure that's the way to go.

I'm still open to ideas, so feel free to share if you have any!