Combine sugar and 3/4 cup water in a small saucepan; bring to a boil. Reduce heat; simmer 5 minutes or until sugar dissolves. Remove from heat; cool completely. Stir in lime juice.
Combine sugar mixture, tequila, orange juice, and amaretto in a pitcher. Serve over ice. Garnish with lime slices.
Spirit note: The most important words in choosing a tequila are "100 percent agave." Unless you spot this phrase on the label, you're getting as little as 51 percent agave, with lots of sugar added to make up the difference. An exceptional value is 1800 Reposado ($24). "Reposado" indicates the tequila was "rested" in oak, imparting flavors of citrus zest and butterscotch, which complement the nutty amaretto in this innovative margarita. --Jeffery Lindenmuth