I think some really underappreciated airports that I find are in Asia are Jeju and Naha (Japan and South Korea). Like in one of my worlds I own about 10% of the service @ Naha with just 1.3 million dollars, but the airport serves 19 million people a year.
Add Bergen airport onto that list as well. Major destinations like Rome and Hamburg left unserved, about 250 demand there, $50000 dollar gate upkeep for just 5 gates and 6 million passengers.
Coming for a relatively unexperienced person.