I have been known, on a hot day, to enjoy a lime flavored frozen margarita from Mexico, but it is nowhere near as good as a nice on the rocks 1800 from the Margarita Bar.
While speaking of margaritas, another great margarita I love is the Presidente Margarita at Chili's. It costs a little more than the 'two-for' margaritas they offer all day, but well worth it. They changed the cheapy margaritas in the last couple of months; they used to be passable, and a nice way to get your drunk on for cheap. Now, they are made with some horrible limade-y tasting mix. And, you can barely taste the tequila (which does have a nice taste if the right tequila is used). Spend the extra two bucks for the Presidente - it comes in it's own shaker, and is basically two margaritas. And it still packs a punch.