If you want a the best margarita, step 1 is to skip the "margarita mix". The drink should be made with freshly-squeezed lime juice.
The best margarita is:
2 ounces tequila
1 ounce Triple Sec or Cointreau
1/2 ounce lime juice
That’s it. Just mix them in a cocktail shaker and pour over ice. Salt on the rim is optional.
Any good tequila will do. I don’t see any point in using an expensive añejo tequila in a margarita; the subtle flavors are lost. It would be like making mixing Coke into an aged, single-malt scotch.
Use a Blanco tequila (a plain white tequila) or a Reposado tequila. Either way, it should be 100% blue agave. Personally, I like the harsher taste of the plain blanco tequila, but a reposado tequila will make a smoother margarita.
The non-reposado Gold tequilas are made with artificial flavors and colors, and they’re just fine if you’re making mediocre margaritas with sour mix, but you wanted a really good margarita.