I do field service and fill up in several states (NY, NJ, PA, CT, DE) about 3-4 times a week at a variety of stations. What I found is it's not so much the brand of gas, but the guy at the refinery (or gas depot) filling up the tanker truck. Apparently it's far from an exact science putting ethanol and the additives in the fuel during manufacturing.
I've gotten great milage from Gulf (go figure, never liked it), BP and Shell are about the same, Phillips 66 and Citgo too.
Here and there I'll get a tank of gas that don't get the mileage, and others that get better. This is on the work vehicle (2015 Ford Escape) with the instant mileage display.