THERE. IS. NO. MARGARITA. MIX. IN. A. MARGARITA.
How "mix" came into it is pretty astounding considering that a Margarita only has three ingredients: tequila, orange liqueur, and lime juice. I guess some marketing genius said, "You know, three steps takes too long. People only need two steps: "mix" and tequila. Plus, we got all that corn syrup laying around...."
From the Food Lovers Companion, a (if not the) definition of a Margarita: a cocktail made with tequila, an orange-flavored liqueur (usually triple sec) and lime juice.
I saw a Margarita recipe once that had all the good stuff in it - and two other things - with one of the other "things" being Sweet & Low.
Sweet & Low?!?!?!
Ingredients matter here as do all recipes when you have less than four of them. For the below the recipe is given as "parts" so you can easily figure out how to scale it. If you did it as "shots" you'd have one knock-you-on-your-back Margarita.
2 parts "good" (silver) tequila (Corazon, Patron, Herradura...)
1 part Cointreau or Grand Marnier
1 part fresh (and I mean FRESH) squeezed lime juice.
Put all ingredients in an ice filled shaker and let sit for about 1 minute. Vigorously shake for about 30 seconds, strain into your favorite salt rimmed glass (or not) and you're done. Garnish with a lime wedge if you've got some laying around.
Watch it with these though. A - it's an expensive drink. If you go and order it as above, you're likely to run up a drink that's going to cost near $20. What's more sad is that your average bartender will look at you funny saying you want a Margarita made this way. But B - this is highly potent. You only need a couple of these before you're having a grand time. So if making them by the pitcher for a party do warn your guests. Otherwise you're going to have what looks like Sangria all over your floor....
You can experiment with this. Say some aren't too fond of tequila? Make the ratio 1:2:1. It's a bit sweeter and still tastes great - I like tequila so I go with the 2:1:1 but for large groups I tone it down. Figure what works best for you and I guarantee you'll never use a mystery mix again.