Based on many years of meticulous data, my best fuel mileage, like T-keith, is on Holiday gasoline. Of course, outisde of Minnesota / upper midwest, you've probably never heard of it. Shell also seems to provide decent fuel mileage in my vehicles.
As to the comment that all gasolines must meet federal spec and should be close, that also would apply to motor oil! As we've seen, there are many ways to acheive the same result, and some work better than others.
In this market, Holiday gasoline can reliably be counted on to be from one refinery, and thus its constency in fuel mileage is not a surprise...