Triple Secs

Plenty of cocktails call for orange liqueur of various manifestations. It would seem that triple sec is a generic term for a dry, colourless, orange-based liqueur, Curaçao describes sweeter liqueurs distilled from the peel of Caribbean Curaçao oranges in particular, and Cointreau and Grand Marnier are specific brands of orangey booze, which depending on the authority consulted either fall into one of the two previous categories or demand their own. It's enough to drive one to orange-flavoured drink (tasting notes below).

Iseo Triple Sec
An Italian triple sec, weighing in at just 15% abv.

Appearance: Clear.
Nose: Orange peel, lemony citrus, soap, astringent.
Taste: Slightly sweet, dryness, violet, floral, orange flower water.
Verdict: Cheap and uncheerful.

Cointreau
40% abv. Produced in Angers, France, from European and South American bitter oranges.
Appearance: Clear.
Nose: Richer, orange, sweeter, zingy.
Taste: Sweet, boozy, orange liqueur.
Verdict: A whole different class to the bog-standard triple sec, but still too sweet by itself.

Grand Marnier
40% abv. Also French, made from a blend of cognacs, pairs well with crêpes.
Appearance: Golden/orange.
Nose: Sweet and rich, brandy, honey.
Taste: Sweet, burnt orange, caramelised, rich, long finish.
Verdict: The grandaddy of orange liqueurs.

If I had to drink any of them straight, with ice, I'd go for Grand Marnier. But that would be missing the point of orange liqueurs, which is to assist in the production of a certain tequila cocktail. More here...