How to choose tequila
If you're looking for a tequila to make mixed drinks like a margarita or a Long Island Iced Tea, where the delicate flavors of fine tequilas wouldn't be appreciated, opt for less expensive silver (blanco, or white) and gold (oro) varieties. They are bottled immediately after distillation, or aged for less than two months before bottling. Gold tequilas are simply silver tequilas that have been mixed with aged tequilas or caramel coloring and sugar so that they take on a light brownish tint.
Aged tequilas are kept in oak barrels before bottling and pick up both color and flavor while they rest. Reposado (rested) tequilas are aged for more than two months, but less than 1 year before being bottled. Añejo tequilas are aged more than one year, but less than three. These tequilas are most often chosen as sipping tequilas and possess nuances than the unaged tequilas don't.
Another factor in choosing a tequila is agave content; according to Mexican law, all tequilas must be made from 51% blue agave sugars, but the other 49% can be corn or cane sugars. Due to this technicality, many tequilas touted as being made from 100% blue agave are significantly more expensive, without a huge difference in taste.
In choosing a tequila for cooking purposes, these are the questions to answer: Are you steaming with tequila? Using it in a marinade? Will the tequila affect the color of the finished dish? Will the smoky flavors of a reposado or añejo even be detectable?
In general, if the tequila is going to be cooked or used with lots of herbs, spices and chiles, then choose good quality silver or gold tequila, as the complex flavors of aged tequilas would be wasted. In highly spiced dishes, the acidity and bite of the tequila should be a bit rough in order to be tasted over the other flavors -- gold and silver tequila, or even mezcal perfectly fit the bill.
In dishes where the tequila will not be heated at all, like an ice cream, ceviche or salad dressing, then an aged tequila could be used. The flavor of a $10 shot of premium tequila will most likely disappear if used in a chili or stew, but may elevate the delicate flavor of lime ice cream to the ethereal.