First a tequila primer... tequilas are sorted by age with the youngest being silver, the middle being reposado, and the oldest being añejo. In general, añejos are more expensive than silvers, as it needs to sit in the barrel longer before bottling.
If you're making margaritas, get a decent silver. Nothing fancy is necessary here ... Sauza Silver is cheap and does the trick. In fact, using a nice tequila for mixed drinks is kind of a waste of nice tequila. If you're sipping it, get a reposado or añejo. You'll definitely taste more of the barrel in an añejo, almost like a whiskey. I prefer reposados myself.
By the way, "gold" tequila, as in Cuervo Gold, is just silver tequila with artificial color to make it look aged. Yuck.
As for suggestions, I like Corralejo. You can get the reposado in the blue bottle, and the añejo in the red bottle. Both versions are smooth for sipping, and won't break the bank. If you're looking for value, stay away from Patron, which is excellent, but well-known, and accordingly overpriced. If you want to shop around, the liquor store at 15th Ave NW and 80th in Crown Hill has a very good tequila selection.