Our Costco has some clear winners, some "pretty good" deals, and some lousy ones.
Offhand, I can think of:
Clear winners:
Kerrygold butter. 3 double-sticks for $7, cheapest I can find anywhere else is $2.99 on sale for one.
Coconut oil. I don't remember the price, but it comes out to something like a third of what it is anywhere else. Organic expeller-pressed etc too.
Rice. I think it was 25 pounds for $9.
Spices. They vary, but they're all super-cheap. I think we got our gigantic container of chili powder for $4 and it lasted us something like nine months, and we are not stingy with our chili powder.
Oranges. Around 40 of them for under $10.
Organic carrots. $7 for 10 lbs, they last forever in the fridge.
Frozen fruit/vegetables. Frozen strawberries and broccoli in particular are cheap there.
Grain-free dog food. Around $30 for 35lbs of salmon, turkey, or beef. Ranks decently as far as store-bought dog food goes, way more economical than comparable products at Petsmart/Petco.
Their supplements/creams/floss/etc tend to be very well-priced too, luckily we haven't needed much from that area lately.
Nuts are pretty cheap there compared to most other places. Have to get a lot though, of course.
Organic blueberries when in season are super cheap compared to everywhere else.
"Pretty good deals":
Gluten-free bread that doesn't taste like butt, and has decent ingredients, $8 for two loaves. Other stores have similarly priced, but taste like butt and generally have questionable ingredients. Or taste good but cost twice as much.
Bacon. They have three kinds of bacon. We get the middle tier one, and it's quite tasty.
Organic eggs. 6.69 for two dozen. We buy ours from a farm, but these are our backup.
We don't get it there, but milk tends to be around Aldi-priced.
Avocados are around six for $5 most of the time.
Fresh wild-caught seafood is usually fairly priced, but it's inconsistently available. Frozen wild-caught salmon is around $11/lb, which is the best I've found around here.
Most of their fresh produce is decently-priced, as long as you actually eat all of it before it goes bad.
Their meat is well-priced for the most part, and a bit above your average grocery store in terms of quality.
Lousy deal:
For some reason garlic there is three times per unit what it costs at our local Meijer.