I know the Margarita request was old, but this is my favorite:

Buy a can of frozen Minutemaid limeade and let it partially thaw.

Blend it in a blender with 1 can of tequila and 1/3 can of triple sec and ice.

YUM!

For any margarita, you want 3 parts tequila to one part triple sec and then some type of sweetened mixer (limeade, fresh juice and superfine sugar, crappy store bought margarita mix, sour mix, etc). This can be blended with ice or just served over it (my favorite) or chilled and served as a martini, with or without salt on the rim of the glass.

As a licensed bartender, my recommendation is to use mid-grade tequilla (the good stuff is for drinking straight!), good quality triple sec (Grand Marnier works well) and the best quality mixer you can find to get a great margarita.