If I'm making
crepes suzette or a souffle, Cointreau and/or Grand Marnier are
de rigueur.
If I'm making margaritas,
any, read cheapest, triple sec (or other orange licquer) is fine. I favor the Steve Dahl recipe, 4:2:1, tequila, fresh squeezed lime, triple sec. Steve's is actually 1.5:1:.5, but I find mine easier to remember.
FYI-I like the packages of key limes that everyone sells now, but they turn before I can use them all. Squeezing and freezing as they turn in little ice cube trays works great.