I was, at one time, extremely current on just about any gas topic out there. I have been out of that business (for the most part) for 10 years now. I keep meaning to sit down and find the studies again that were the most detailed brand-by-brand comparison, but...
To summarize, basically all of the "name brand", non discount chains scored much higher than the places that sell "cheap" gas. Your Amoco's(BP), Sunoco, Shell, etc all scored very well. The discount chains scored lower.
The best advice I can give is this - start with a clean system and chuck over the extra 2-3 cents to get name brand fuel from then on. There is also a fun little phenomenon that occurs when you use shit gas. Over time, the build-ups actually raise your compression ratio, eventually forcing you to run higher octane fuel to prevent pre-detonation.