I'm certainly not an expert, but my rule is to to use a decent (100% agave) blanco for margaritas, along with Cointreau. The last two basic margarita tequilas I bought were Herradura Silver and Plata Hornitos and we were pretty happy with the results. I've paid more for tequila and tried other brands, but there are many to choose from and it can be a matter of personal taste. Next bottle I might try something different.
I like a bright citrus-forward margarita and think the blanco supports that best. With a reposado tequila margarita, I am more likely to balance that flavour with Grand Marnier instead of Cointreau.
I often add a small splash of fresh orange juice along with the lime juice. And I like Prickly Pear Margaritas but I only use the prickly pear juice or nectar, never the prickly pear syrup which I think tastes awful.