I like to buy gas from Shell, but any of these companies should be fine too:
MFA Oil Company
Kwik Trip/Kwik Star
The Somerset Refinery, Inc.
Tri-Par Oil Company
They are all "Top Tier" rated and have agreed to have greater levels of detergents in their fuel that currently required by the EPA, and such will protect and clean you engine better then a fuel with only the minimum level of detergent.