Unfortunately, most commercial versions of this essential flavoring contain little to none of the ingredient that gives it its name: pomegranate juice (“grenade” is French for “pomegranate.”). Angostura and Monin make decent syrups, but you can make your own, and it will be better than either.
Combine equal parts (by volume) pomegranate juice and superfine sugar. Shake until the sugar is dissolved. Alternatively, heat the mixture very gently, stirring often. Heat only enough to dissolve the sugar, and remove from heat immediately.
Add one ounce of high–proof spirits (e.g., 151–proof rum) per quart of syrup. This prevents mold.