ORIGINAL: RoswellGrey
In San Antonio, Shell seems to be the best. Valero (which is virtually everywhere, since the HQ is here), varies from tankful to tankful -- sometimes it knocks, sometimes it doesn't. I also use Exxon from time to time. It's about halfway between Shell and Valero.
Actually, the best gas mileage I've ever had has been on the Kansas Turnpike. I was able to get 32 MPG out of my '94 Thunderbird there (it usually got 28-30 on the highway) and 29 out of my '00 Mustang (usually 25-27). The surface of that road is incredibly smooth -- as it should be, since it's fairly costly to drive it. If anything, I'd say road surface is the single biggest difference (excluding wind, of course) there is when it comes to mileage.
We have Shell, BP, Phillips 66, Conoco, QT, Mobil, and Murphy USA (if you consider that to be a major name brand since it is found only at Wal-Mart) as our major name brands in this area. I haven't had as good luck with Shell, though I know lots of people who swear by it. BP is okay. Phillips is one of my back up places to go due to good results. I haven't had much experience with QT. I've never heard of Valero before so I'm guessing that is a Texas area thing. A few local chains we have in this area is Casey's (gas is lower grade but some items in the store such as donuts and pizzas are actually pretty good), Huck's (it's like Casey's without as good products and I never go there), Handee-Mart (this chain is becoming rare and for good reason, lower quality fuel), Gateway F/S (a farm co-op, farmers and truckers often go here, has E10 and/or E85 available consistently, many mechanics I know recommend it), MFA Oil (on the Missouri side another farm co-op from what I understand, just a Missouri version from what I understand, it was listed on the top tier fuel list so that says something), Moto Mart (middle quality fuel, has large quantities of items in their store), ZX (a lot of Citgos changed to this and I have never been there and have no plans on going there either), and a few other independents.
The absolute best I have ever received with my car is 36 mpg a few times with pure highway driving. 32 is my average on the highway.