EdE, I like to use Sauza Hornitos or, when I can find it, Siete Leguas - both reposado. Both are of high quality and reasonably priced; for my budget at least. I'm also a proponent of Cointreau over Triple Sec or Grand Marnier and fresh lime juice and a bit of simple syrup. I'm much more of a "mix by eye" mixologist but years of tending bar during my college days result in good consistency when I'm pouring. When I've been forced to use instant mix, Cuervo Marg Mix, (the only Cuervo product I like) with just a tiny splash of orange juice results in a passable margarita, especially when pouring for a large crowd where juicing limes would occupy the greater part the day better spent on meal preparation.