Costco always lowest price. Sams will match them when nearby--usually like to stick to Top Tier so generally avoid Sam's.
QT (Top Tier) dominates mainline c-store gas sales in metro St. Louis and has served as the rabbit in the rather odd gas market here. QT down three cents (everyone follows), down seven cents (everyone follows), up thirty cents (everyone follows within an hour or so). This was so strange that it drew the price fixing ire of the AG about a decade ago, but nothing came of it. QT got the jump on the market about 25 years ago because they had large, safe, well lighted stores-- heavily staffed 24/7 and cops were encouraged to use them as informal sub stations. Today they have separately staffed lunch counters with 49 cent soft-serve, on order pizzas and flat breads and breakfasts. Can't say they're cheapest because other than an hour when they cut prices, everyone else in the market goes along for the ride. Most gas here is Top Tier, On the Run (Mobil) and QT (Shell) make runs at QT and also have nice stores. Now we need a Bucees to take things to the next level-- I'd go in there for BBQ and to look at the giant rows of urinals but don't think their gas meets Top Tier.
Ninety percent of my actual purchases are Costco. When QuikTrip gets within a few cents of Costco, you know they're going to jump prices by a quarter within a day or two.