To be perfectly honest, there's not a great answer to this question. I'll do my best though.

Produce is a no-brainer. Even the huge bag of carrots. And here's why: in most cases, even if you throw out half of the bag, you're going to break even or be ahead, relative to supermarket pricing. So unless you have a big issue with moral irresponsibility, you won't find a better source of produce.

Costco actively maintains a lowest price standard. What I mean by that is, when you compare apples to apples, Costco will always have the best price. If that becomes untrue for some reason, the item will not be made available for sale until we can figure it out. This is intended to assure our members that they are getting the best deal on their purchases and save them the time of having to comparison shop. Of course, most people still do. It's only human. But in my personal experience, I've almost never seen that fail.

Foods (dry goods, refrigerated, frozen, prepared) are all an incredible value. Non-foods (clothing especially, media, hardlines) will be priced the lowest you will see anywhere.

The only exception I can even entertain is electronics. With Internet shopping being what it is today, there are an unbelievable amount of hot deals going on at any given time in every major category of electronics. On a day to day basis, I stand by the statement that no one will beat Costco's price in a direct comparison, but for electronics, I will allow that one might get lucky to the tune of a few bucks here or there.

I realize this answer has been entirely concerned with value up until this point, which I took as the main thrust of your question. If we're talking about practicality, though, I suppose the only thing you really need to know is this: if you're going to buy something, make sure you'll get the right amount of use out of it, otherwise that all-important value will be lost on you, and this whole argument will be moot.

My wife and I actually go through that big bag of carrots every week or so. If you're only eating one carrot a day though, maybe get the small bag at Stop & Shop.