Frozen or on the rocks? That question can really only refer to one thing: the margarita. And cocktail enthusiasts have learned to turn their noses up at the frozen variety, but I'm here to say that that's the wrong move.
I'll repeat: Putting a margarita on the rocks is about the worst way to drink the cocktail. (And putting salt on the rim? I have more to say on that below.) A classic margarita is a shaken cocktail; the chilling and diluting that occur by shaking tequila, lime, and triple sec with ice defines the drink. Pouring that drink over ice will make it watery and wan.
It's not hard to understand the place of the frozen margarita in this culture: A slurry of ice and liquid attains a colder temperature than a drink served on the rocks. So much so that caution must be used to avoid the dreaded "brain freeze."
I never drink slushy margaritas (or slushy anything, really) except when I'm home in Texas, and then I drink a lot, because they're delicious. This Slushy Blended Margarita is pretty close to the ideal. I would suggest that you use Minute Maid Limeade, though, as it's well-balanced between sweet and tart, and that you squeeze in a whole lime or two for added bite.
The best way to have a margarita is the most classic, but also the most rare: elegantly served "up" in a martini glass. Most cocktail historians say this is the way the margarita would have been served originally (likely in the ’30s or early ’40s somewhere along the U.S.-Mexico border, probably in Tijuana). Presented this way, the mouth-watering lime aroma and the exotic, herbal/sweet scent of distilled agave really comes through. The secret to making a great "up" margarita is (naturally) freshly squeezed lime juice, good tequila, Cointreau (or agave nectar), and a very good, prolonged 10- to 12-second shake for aeration and dilution.
Do it properly, not too tart or too sweet, and serve it "up" in a chilled martini glass—a glass that won't stay chilled for long in a hot climate like Texas. So, if it's 95 degrees out, you might just want to go with something frozen.
And as for that salt question (again, cocktail purists look down their noses at such things), it's up to you. Personally, I don't find that salt clouds the flavor of a margarita. In fact, a couple of grains on the tongue can actually help bring it out. The solution is simple: Don't salt the whole rim, and then everyone will be happy.