This is one of those drinks that everyone has a recipe for. This version is my favorite by far, despite the effort required (squeezing limes). The easy way to remember this one is 4:2:1 or 4 parts tequila/mezcal, 2 parts lime juice, 1 part agave syrup. Sauza or Hornitos are both great tequilas to use for this. If you're going with mezcal, try Del Maguey Vida. You can replace the agave syrup with Cointreau (leave out the Gran Marnier) for a more rounded citrus flavor (and more ABV). Also, if you're feeling frisky, muddle a slice or two of jalapeno in there before adding the ice (take the seeds out first or it might be undrinkable).
silver or reposado tequila or mezcal
fresh-squeezed lime juice
Gran Marnier (optional)