The Serious Eats Field Guide to Orange Liqueur
from https://www.seriouseats.com/, by Michael Dietsch

Before diving into a tasting of various brands, I should start with some terminology. It's difficult to pin down precise definitions of these terms, but I'll try. Orange liqueur is the easiest term to define, so here's where I'll start. Orange liqueur is simply a sweetened alcoholic beverage with orange flavoring. The alcohol itself can be either a column-distilled neutral spirit (similar to vodka) or it may be a pot-distilled spirit, such as grape brandy.

Curaçao

Historically, curaçao is a product of the island of Curaçao, made from a pot-stilled brandy and flavored with the dried peels of Curaçao oranges. These oranges were first brought to Curaçao as Valencia oranges by Spanish settlers. But the Valencia orange didn't take well to Curaçao's dry climate, and over time, the oranges became bitter and inedible. The plants began to grow wild, but then someone—it's unclear who—discovered that the skin of a Curaçao orange, left to dry in the sun, produced a pleasantly fragrant aroma. By 1896, distillers in Curaçao were using the peels of the Curaçao orange to add flavor and aroma to their distilled products.

Curaçao is not a protected appellation. Some products, such as cognac and Champagne, are protected, and must by law be made in their region of origin. Curaçao doesn't enjoy this level of protection, however, and therefore it may be made anywhere and by any method. Earlier, I said that historically, curaçao is a product of Curaçao, made from brandy and dried Curaçao orange peels. Modern curaçaos, however, are not necessarily made this way.

Some traditional curaçaos are still available, most notably, a brand called Curaçao of Curaçao, produced by the Senior family of Curaçao. The blue stuff, I probably don't need to add, is not a traditional curaçao.

Triple Sec

Triple sec originated as a French product. It was originally made with less sugar than used in curaçao, which led to the name sec, which means dry. No one knows with any certainty where the designation "triple" originated. Triple sec is not triple-distilled, as some people claim, and nor is it thrice as dry as curaçao or other liqueurs. The most reasonable explanation is that "triple" was mere marketing, a way to trump up new products and denigrate the competition.

What's the Deal with Grand Marnier and Cointreau?

Perhaps the two most famous brands of orange liqueur are Grand Marnier and Cointreau, and you might be wondering where they fit in to this taxonomy. Simple. Grand Marnier is an orange liqueur in the curaçao tradition, and Cointreau is a triple sec.

Grand Marnier is a blend of cognac and triple sec, so although it's not a traditional curaçao, it's a similar product. Cointreau, on the other hand, is straight up a triple sec. In fact, Cointreau initially called itself Cointreau Triple Sec, and you can sometimes find old-school, collectible bottles with this labeling on Internet auction sites.

In response to the dreck that other distilleries were putting out and branding as triple sec, Cointreau repositioned itself as a product apart from triple sec, removing those words from its label and marketing.

Grand Marnier is a clear winner as a standalone, after-dinner sipper. The flavors are beautifully balanced and the palate is dry overall, and not very sweet. As a mixer, GM can be tricky. A lot of bartenders find it too brandy-forward for Sidecars, for example, skewing the flavor profile of such cocktails too heavily spiritous. I urge you to experiment for yourself, and enjoy the endeavor, but just be aware that you'll need to take a light hand with Grand Marnier. You can always add more.

Cointreau is well-regarded for a reason. The flavors are perfectly balanced between bitter orange and sweetness. Warm spices lend complexity to the spirit while complementing the orange. In a great orange liqueur the other flavors should serve to enhance the orange flavor, not mask it, and in this regard Cointreau excels. It's not my choice for a pure after-dinner sipper, but it's tops for cocktail versatility.