For an extra delicious, classic margarita, I use

1 oz layenda del milagro tequila blanco
1/2 oz cointreau
1 tablespoon of agave syrup
And as much fresh squeezed lime juice as you like!

I mean really, the strength is up to you. I like mine medium-weak, because otherwise I'll be a wreck.

In my opinion, it's the agave syrup that really makes it.