We taste-tested 12 brands of margarita mix you're likely to find at your local supermarket or online. To find the very best ones, we tasted each without knowing which was which, mixed with blanco tequila. Our winner is Fever-Tree Light Margarita Mix , but we also named one worthy runner-up. A margarita is made with blanco tequila, triple sec, and fresh lime juice—and maybe some simple syrup, if you aren't using high-quality tequila. Bottled, nonalcoholic margarita mix should then, in essence, be a blend of lime juice, simple syrup, and some kind of orange flavoring, like extract, to mimic triple sec.  Seems simple. Scan the many options on market shelves, though, and you'll find a dizzying array of product types and formulations.…