Retire early: You want to save as much money as possible. So, low cost of living (rent, food, general life expenses) and low flight costs.
Frequent flights: Visiting South America once or twice a year. US once or twice a year. EU a few times a year.
I've been looking at international flight options on Google Flights and Kiwi from different cities in last 6 months in detail. I determined the cities based on busiest international airports in the world, the data on numbeo and my own research. My conclusion is the best city in the world for my purposes is Istanbul. Is this correct?
If so I'll move to Istanbul. I'll be there for about 6-8 months of the year.
Someone voted that this question is opinion based. No, it is not opinon based. Data is clear on Google Flights, Kiwi, Numbeo and Wikipedia.