If you're a Costco shopper, you're familiar with Kirkland Signature.

Selling everything from diapers to bedsheets to hair regrowth serum, the chain's private label brings in nearly a quarter of Costco's revenue. It has become beloved by fans since its debut in 1995 not only because of its prices, but because its offerings are good enough to compete with national brands.

Few know this better than David and Susan Schwartz, authors of "The Joy of Costco". While researching their book, the couple visited more than 250 warehouses, including locations in the 13 countries outside the U.S. that have Costco.

"We've tried Kirkland Signature around the world," Susan tells CNBC Make It. "They're doing a great job. The products are truly as good or better than the branded items, and the prices are always really killer." 

The pair hold a special place in their hearts for Kirkland, which, to them, is one of Costco's biggest draws.

"The thing about Kirkland Signature is it really gets to the heart of what Costco is trying to do, which is to build loyalty and maintain a level of trust with its members," Susan says. "I've never felt like I'm being taken advantage of. I've never felt like I could find it somewhere else for a better price." 

David and Susan both have their favorite items from Kirkland Signature's more than 500 offerings. Susan is fond of the brand's workout leggings, while David picks up a few button up business shirts each year. They love the bacon crumbles, organic peanut butter and call the Kirkland Signature hot dog "a thing of beauty."

But there's one Kirkland Signature offering that they say stands out from the rest.

Their favorite Kirkland Signature product: Booze

Costco is one of the biggest wine sellers in the world. And for David and Susan, the chain's selection is unmatched. 

"They put the Kirkland Signature brand on some of the finest wines you can buy and offer them for unbelievably good prices compared to what you'd get if the chateau were slapping its own label on the bottle," David says. 

He recommends trying the Kirkland Signature Barolo or Bordeaux, which he says he picks up from Costco for half of what he'd pay for a comparable bottle at his local New York City wine shop. 

Susan, meanwhile, swears by the store's in-house liquor selection. So much so that the former devoted Tanqueray drinker has shifted her allegiances to the cheaper Kirkland Signature London dry gin. 

"When we're having a party or we need to stock up, we rent a car and drive out to Wayne, New Jersey because they have a huge liquor section," she adds. "You can get the Kirkland gin for around $17, and it's the same size as a bottle of Tanqueray."

The car rental and hour long drive might seem like a lot to save money on booze, but Susan says it's worth the hassle.

"Kirkland Signature liquor," she says, "is spectacular."

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