This map shows every country in the world and their requirements for Overflight Permits and Landing Permits. For overflying aircraft, the yellow countries will want  you to have a permit, and the black ones don’t. That’s for routine flights at least, if you’re on a Special Airworthiness or missing an engine, then pretty much everyone will want one.

Click on a region and you’ll get that …

And then click on the individual country to figure out what kind of overfly clearance you need.