This is a difficult question, because many members have fierce loyalties to one brand or another. Right now, the only brand my killies will eat seems to be soldier fly based bug bites flakes, or pellets run through a grinder. But when I had many tanks of livebearers, rainbows, cichlids, rasbora and others, I bought bulk flakes. I got them from several sources, who I think all got it from the same source. I bought 500 gram bags, usually three types at a time. I removed small amounts, and froze the rest.
Now, I have been buying brand name flakes, of several brands, and really, I see no difference with it, and the infinitely less expensive bulk product I used to buy. Growth rates, colour, overall health, breeding - nothing has changed except the number of flake eating fish I have.
This goes against the advice I see here, but it is my experience.
Whether I bought in Canada, or picked things up when I was visiting the US, I seemed to get the same product from the more inventive mail order places. I haven't ordered for a few years but ken's fish was good. I'm sure there are many others.
The issue is how many fish you have. A pound of flake will be stale long before someone can use it unless they have a huge set up. It begins to degrade, vitamin-wise, after less than a year. The difference isn't huge, but it does happen.