Cointreau is probably the best known triple sec around, triple sec meaning "triple distilled" is a term for orange flavored liqueur. If you are to buy just one liqueur, you might want it to be triple sec as it's extremely useful in the bar.
Absolut Vodka, Lemon Juice, Saffron Sugar Syrup, Cointreau, Orange Juice
Absolut Apeach, Cointreau, Honey, Apple Juice (Hot), Orange
Sherry, Cointreau, Orange Juice, Orange Bitters