What is the best all purpose flour to use for baking?

I use King Arthur (even in Stella Parks recipes) unless a recipe specifically calls for something like bleached cake flour (which isn't even AP) or gives a specific reason to use bleached flour. I've used Bob's Red Mill, and also whatever others have around, like if I'm baking for family when I visit, and have never been like "Oh no! This turned out terrible, it must be the flour brand!" AP generally is AP, regardless of brand.

Not really an answerable question. There is a big difference in protein content- KA is 11.7%, just short of being classifiable as a bread flour, and I prefer it for pizza crust. Bleached flours are in general lower protein and are generally preferred for pastries, cookies and cakes- though it's not a startling difference. Rose L. Beranbaum, who I'm not inclined to doubt, tells us that bleached white will also brown more. As far as brands, Gold Medal is everywhere and is dependable, but I have no real basis for comparisons.

Any good organic AP flour will work well for you. Bob's Red Mill products are always reliable, and generally widely available.

Stella Parks, the author of Bravetart, says she likes Gold Medal (see here: https://www.seriouseats.com/2018/10/basic-essential-baking-ingredients.html). I use King Arthur. I don't bake anything super fussy, though, so I haven't noticed a huge difference.