Flour or corn starch for thickening pie filling?

I think it is going to depend on the pie that you want to make. Here's a nice guide to the various thickeners:
https://www.thekitchn.com/5-common-pie-thickeners-and-how-they-work-baking-guides-from-the-kitchn-212793

And King Arthur Flour (my baking bible) did some testing (with great pictures!) on fruit pie fillings and some of it is eye-opening: https://blog.kingarthurflour.com/2013/06/21/fruit-pie-filling-no-runs-no-drips-no-errors/

...or, no doubt, potato starch, corn meal or any other sort of starch. I use flour for apple pie because that's the way I learned it from my mother and cornstarch for berries and cherries because I always have it, but I don't see it as much of an issue.

Or arrowroot

You can also use tapioca

People use either but I prefer corn starch.