We fill up at Kroger, Walmart and Sam's whenever we can, just using our cards.
Their prices always beat whatever else is around.
Gas stations owned and operated by American Indian tribes/casinos frequently have markedly lower prices, too.
That said, part of our daily plan of attack to travel inexpensively is to watch diesel prices and snag a top off when we see a good price, even if it is just a quarter of a tank. Always a good excuse to stretch our legs, give Miss Lily a pee-and-sniff opportunity, etc.