Sauza Hornitos Tequila
It's probably a good idea to use a decent tequila in your margaritatini since the there's no ice to dilute the bite if one were to risk using some rotgut. "Rotgut." A good word. A word my mother would use, which is funny because it's even too old fashioned for her. But she's good at keeping arcane bits of the English language alive. Which is has everything to do with decent tequila... because, in both cases, life's too short to compromise oneself with the banal.
Smooth, inexpensive, mixes well in margaritas