I consider myself a decent amateur baker and I've never noticed a huge difference in bread flours. I buy from a local mill because they sell interesting grains and I want to support them. Otherwise I get King Arthur, or Bob's Red Mill if I need something a bit more specific. KA is particularly good because their recipes are usually reliable and they're designed around their flours. The selection of baking ingredients is usually much wider in the average US supermarket compared to the average European one in my experience.