I am in the gasoline business. And even though I don't own a Shell Station; The best gasoline to put in your car is Shell. The reason: They usually own the land. And hence they control what GOES into those tanks.

After Shell:

#2 Exxon

#3 Cheveron

#4 Everyone else is equal..

An easy way to determine if a gas station sells good quality gasoline is to find out who owns the tanks/land. If it's a gasoline company then it's a good bet that the gasoline is of 'normal' quality.