This is adapted from joanie's recipe, see hers for more swell notes on making a margarita. This recipe is the proportions I choose, and is scaled to a single drink.

Once you've got the proportions about right, the next step is selecting your liquors. For the orange liqueur, quality really makes a difference. The gold standard is Cointreau, but it's really quite spendy. One quality/price step down from that is appropriate, but if you're reaching for a Triple Sec, you've fallen too far. Grand Marnier is often used in margaritas, but it's an orange brandy rather than an orange liqueur, it's a different flavor and not an appropriate swap for this recipe.

The trickier part is the tequila. My general guideline for selecting a good base spirit applies here: look for a brand that is familiar to you because they spend a lot on marketing, note its price, then select a *different* brand that does not spend a lot on marketing, at that price point or slightly above. It doesn't matter if you've never heard of it at all. Stick to 100% agave tequilas. The barrel aging in tequilas goes silver/blanco (youngest) → reposado (medium) → añejo (oldest). I generally reach for a reposado when making margaritas.