Like many have said, the cheapest.
In my area that is usually Thornton's, Murphy's (when you use some sort of Wal-Mart card to get the $0.03 discount) and Meijer (with their card, a $0.05 discount). However, there are some BP and Shell stations that are sometimes cheaper, all depends on who gets what delivery when, I guess.
Also in my area are Exxon/Mobil, Citgo, and 7-11. I'll use the 7-11 and XOM stations sometimes, but I can't bring myself to go to Citgo. I'd explain, but that would get political.....