This is a hard question. I'm brand-loyal to few things. Almost everything is acceptable to me under any brand:
I wouldn't eat mayonaise or margarine of any brand -- disgusting products both. I prefer Peter Pan peanut butter, but will accept other brands. Ditto for catsup, eggs, spaghetti sauce, popcorn, milk, bread, and dozens of other things.
Some things are "borderline acceptable"; for example, soup. If I know I'm mixing it into a casserole, store brand's fine, but if we're eating it as soup, I'll pay a little more for Campbells. But I dislike Progresso, which costs more, and I won't buy it at any price. I usually have both name-brand and store-brand in my pantry, and I'll choose based upon what I'm doing with it.
Diet Coke is one of my must-haves. I don't drink that much of it, but Diet Pepsi or off-brand is not the same. I only use Tenda-bake pancake mix, which isn't all that easy to find; I 'specially like the stuff with maple bursts, but my kids like blueberry. If I make a boxed cake, it must be Duncan Hines -- though Betty Crocker makes good stuff in other categories. And I always have Bounty pick-a-size paper towels.
I can't think of any other examples.